The hind part of the neural plate in amphibian embryos has a mesodermal significance although it occupies an ectodermal position till late neurula stage. In anurans it gives rise to the posterior tail somites (Smithberg, 1954) and in urodeles the somites of the posterior trunk region and of the entire tail together with several other mesodermal structures (Bijtel, 1931; Nakamura, 1938; Aufsess, 1941; Spofford, 1945; Chuang, 1947; Ford, 1949). Presence of mesoderm in the neural plate is an interesting developmental problem. During normal gastrulation this region is at first underlaid by the future anterior part of the archenteric roof which exerts a neuralizing inductive influence. It is only later that the posterior part of the notochord with its mesodermalizing influence comes to lie under it. According to Eyal-Giladi (1954), who worked on gastrula stages of the Axolotl, even a short and transient contact of the invaginating archenteric roof with the overlying ectoderm produces archencephalic induction in the latter. It is therefore relevant to ask whether the posterior part of the neural plate does or does not pass through a phase of neuralization before assuming mesodermal properties. This specific question has not been investigated although a number of workers have studied the determination, differentition capacities or inductive properties of the caudal portion of the neural plate in amphibians (Bytinski-Salz, 1931; Bijtel, 1936, 1958; Chuang, 1947; Nakamura, 1947; Spofford, 1948; Smithberg, 1954). Spofford (1948) demonstrated that in Ambystoma mexicanum neurulae the tail notochord induces the overlying ectoderm to acquire mesodermal properties. He replaced the posterior part of the neural plate by a piece of competent ectoderm which itself had never been subjected to the influence of a neural inductor. This technique could demonstrate that, under experimental conditions, the so-called transformation or mesodermalization need not be preceded by activation or neuralization. These results, however, do not show whether this is also true during normal embryogenesis.

According to Nakamura (1947), the presumptive tail somite region of the neural plate even from an early neurula stage is able to differentiate into muscle tissue if grafted together with prospective tail neural tube. This finds support in the results obtained by Takaya (1959) who explanted as sandwiches the neural plate as a whole or in parts at various stages of neurulation. He found that from the beginning the neural plate possesses self-differentiation potencies which manifest themselves even in the absence of the underlying mesoderm. These potencies are of two kinds: (a) that for neural differentiation existing along the whole length of the neural plate and (b) that for muscle differentiation restricted to its caudal end. Takaya had worked on Triturus pyrrhogaster, Hynobius nebulosus and Rana nigromaculata. Smitherberg (1954), on the contrary, has emphasized the importance of the presence of the notochord to enable differentiation to proceed in somites from the posterior neural plate in Rana pipiens. If it is correct that definitive cytological and histological determination of the prospective tail somite material in the neural plate does not occur until late neural stage and that it is caused by the underlying tail notochord, then it appears surprising that a combination of the prospective tail somite and spinalcord materials from even the earliest neurula stage should enable the former to differentiate into muscle tissue. The present investigation was therefore designed to ascertain the differentiation capacities of the prospective tail somite region of the neural plate from early to late neurula stages of Ambystoma mexicanum when explanted alone or in combination with spinal cord and/or notochord.

Early, middle and late neurulae of Ambystoma mexicanum at Harrison’s stages 13, and 17-18 were used in this study. Chuang’s (1947) maps of presumptive regions of tail organs were followed to locate the areas to be isolated from the neural plate at various stages of neurulation. The general procedure consisted of cutting out the specific regions from the neural plate with the help of glass needles and hair loops and explanting the isolated parts, sandwiched in pieces of ectoderm taken from the mesoderm-free anterior belly region of stage 12-13 embryos of the same species. Seven different experiments were performed; they were numbered as series I-VII. The details of procedure followed in each series are described in the text and are also presented in Table 1. Figure 1 shows the location of the presumptive regions of tail tissues isolated from the three neurula stages used in this investigation. A 20 x 20 (1cm2) ocular grid was used to cut out exactly the same region(s) of the neural plate in each embryo. The figure also shows the manner in which each embryo was oriented in relation to the grid during the operation.

Table 1

Tissue differentiation obtained in explants of the prospective tail somite and spinal-cord regions of the neural plate of Ambystoma mexicanum neurulae, made alone or in combination with notochord

Tissue differentiation obtained in explants of the prospective tail somite and spinal-cord regions of the neural plate of Ambystoma mexicanum neurulae, made alone or in combination with notochord
Tissue differentiation obtained in explants of the prospective tail somite and spinal-cord regions of the neural plate of Ambystoma mexicanum neurulae, made alone or in combination with notochord
Figure 1

Semi-schematic drawings of Harrison’s stage 13, 1412 and 17 embryos of Ambystoma mexicanum. The outlines were drawn by camera lucida at x 25 magnification. The central part of an ocular grid is superimposed at the same magnification to indicate the orientation of embryos and the parts isolated during operation. TS, Presumptive tail somite; SC, spinal-cord regions of the neural plate; NC, tail notochord.

Figure 1

Semi-schematic drawings of Harrison’s stage 13, 1412 and 17 embryos of Ambystoma mexicanum. The outlines were drawn by camera lucida at x 25 magnification. The central part of an ocular grid is superimposed at the same magnification to indicate the orientation of embryos and the parts isolated during operation. TS, Presumptive tail somite; SC, spinal-cord regions of the neural plate; NC, tail notochord.

The sandwiches were cultivated for 11-13 days in Holtfreter’s solution on agar underlay. Penicillin G (20000 i.u./l.) was added to prevent infection. The medium and agar underlays were changed every 2 days and the explants cleaned of extruded cells with a hair loop. At the end of cultivation the explants were sketched by camera lucida and fixed in Smith’s fluid. They were later sectioned at 7-8 μ thickness and stained with Mallory’s triple stain or haematoxylin and eosin.

The donors of the prospective tail materials were also reared for an equivalent period after which they were sketched, fixed in Smith’s fluid and their posterior parts sectioned for microscopical examination to ascertain if removal of specific tail primordia caused any corresponding deficiencies in them.

The results of microscopical examination of the explants of all seven experimental series are presented in Table 1.

Series I

In this series the prospective tail somite region was cut out from stage 13 embryos and explanted alone in an ectodermal sandwich. After cultivation for 11-13 days no muscle differentiation was found in any of the fourteen surviving explants. On the contrary, neural tissue had differentiated in nine cases. These included seven cases of good neural-tube formation, one of which is shown in Fig. 2 A. In the other two explants there were many melanophores together with some groups of cells whose nature could not be identified. Many of these cells were degenerating and contained pycnotic nuclei. Since melanophores are of neural origin these two cases are also considered as those of neural differentiation. In the remaining five cases connective tissue alone or with masses of degenerating cells was found.The examination of the donors of the explanted prospective somite region confirmed that the isolated part of the neural plate did consist of the tail somite material, for in thirteen out of eighteen donors the tail was either very small or totally absent. The remaining five donors had somewhat better tails but all were deficient in muscles. Several of the short tails had no muscles at all.

Figure 2

Abbreviations’. N, neural tube; NC, notochord; S, somites; V, vesicular structure found in serial sections to be continuous with the neural tube. A. A section of a series 1 sandwich, cultivated for 13 days. Presumptive tail-somite region of the neural plate of stage 13 embryo was explanted alone. Only neural tube differentiated, x 100. B. A section of a series 111 sandwich, cultivated for 11 days. Presumptive tail-somite region of the neural plate of a stage 17 embryo was explanted alone. Only connective tissue was formed, x 100. C. A section of a series III sandwich, cultivated for 13 days. The operational scheme was the same as in the case shown in the previous figure, x 100. D. A section of a series V sandwich, cultivated for 12 days. A strip of the neural plate of a stage 141 embryo including presumptive tail spinal-cord and tail-somite region was explanted, x 100.

Figure 2

Abbreviations’. N, neural tube; NC, notochord; S, somites; V, vesicular structure found in serial sections to be continuous with the neural tube. A. A section of a series 1 sandwich, cultivated for 13 days. Presumptive tail-somite region of the neural plate of stage 13 embryo was explanted alone. Only neural tube differentiated, x 100. B. A section of a series 111 sandwich, cultivated for 11 days. Presumptive tail-somite region of the neural plate of a stage 17 embryo was explanted alone. Only connective tissue was formed, x 100. C. A section of a series III sandwich, cultivated for 13 days. The operational scheme was the same as in the case shown in the previous figure, x 100. D. A section of a series V sandwich, cultivated for 12 days. A strip of the neural plate of a stage 141 embryo including presumptive tail spinal-cord and tail-somite region was explanted, x 100.

Series II

In this series twelve explants of isolated prospective tail somite region from the neural plate of stage embryos were cultivated as sandwiches for 12-13 days. All survived but muscle tissue did not develop in any of them. In nine only connective tissue, with or without degenerating masses of an unidentifiable type of cell, was found. Figure 2 B shows a section of one of these explants. Neural differentiation was found in three cases, including two of neural-tube formation and one in which only melanophores were present.

Among the donors of tail somite material seven did not develop a tail of any appreciable length. In the remaining five cases the tails were much smaller than normal and had little muscle tissue.

Series III

In this experiment stage 17-18 embryos were used as donors of the prospective tail somite material to be sandwiched alone in ectodermal pieces and explanted. Out of fifteen sandwiches thirteen survived cultivation for 11-13 days, but in three of them the tail material was probably lost and only empty epidermal bags resulted. Analysis of the remaining ten showed the presence of melanophores and connective tissue in one case, a small neural tube in another and only connective tissue with undifferentiated or degenerating cells in the rest. The sandwich containing neural tube also contained a large vesicle (Fig. 2C), whose lumen was continuous with that of the neural tube. This structure could be an enlarged part of the neural tube. The cells of its walls did not look like myoblasts. Cords of cells of an unidentifiable type and arranged in a similar manner around a large cavity were also observed in a few other cases belonging to series V, described below.

Nine donors of the explanted somite material did not develop tails at all or possessed very small tails. Musculature was deficient in the tails of the remaining six also; their tails were smaller than normal and also abnormal in shape.

Series IV

Stage embryos were used in this series. A strip of presumptive tail spinal-cord material was isolated from the neural plate and sandwiched alone between pieces of ectoderm. Out of twelve explants cultivated for 13 days eight survived and in six of them neural tubes were formed. Only connective tissue was present in the remaining two cases, in one of which capillaries and some blood cells were also found.

The donors of the explanted spinal-cord material developed more or less normal tails in nine out of twelve cases. The tails were often curved upwards and the ventral fin was not well developed. The tail was very defective in three cases; corresponding explants from two of these donors did not have neural tubes. It is possible that in these cases the operation had not succeeded in isolating the spinal-cord region of the neural plate. Instead, some presumptive somitematerial may have been cut out by mistake, which may explain the defective tails in the donors and the absence of neural differentiation in the explants.

Series V

A total of sixteen explants of neural-plate material from stage embryos were cultivated for 13 days in the usual manner. In each case a rather broad strip of the neural plate was cut out and sandwiched between ectodermal pieces. The isolated strip consisted of presumptive spinal-cord and tail somite regions. Out of ten survivors nine contained good neural tube formations. Their lumen was generally narrow but enlarged at places. In five cases packed mesenchymal cells were also seen but there was no sign of either segmentation or myofibril formation to indicate any degree of muscle differentiation. In two cases cords of cells were found around large cavities as was seen in one case of series III, described earlier. Sections of such examples from this series are presented in Figs. 2D and 3A. In both cases the neural tubes were continuous with these vesicular structures. One explant contained only connective tissue and degenerating cells with pycnotic nuclei.

Among the donors of the neural-plate strip the post-cloacal projections consisted largely of fins of abnormal shape. Four donors developed short tails with small somites.

Series VI

In this series also a broad strip of the neural plate including prospective tail spinal-cord and somite regions was cut out from stage embryos. This strip was combined with a piece of notochord from below this region of the same embryo. A total of thirteen such combinations were sandwiched, each between two pieces of belly ectoderm from another neurula, and cultivated for 12-13 days. Five explants were lost. In seven of the surviving eight cases neural tube, somites and notochord differentiated and showed definite tail-like organization. One of them is shown in Fig. 3B. The remaining sandwich had broken into two pieces whose sections showed a neural tube in one and groups of coalescing myoblasts in the other (Fig. 3C). It seems that the notochord was extruded as the sandwich broke into two fragments.

Figure 3

Abbreviations:. N, neural tube; NC, notochord; S, somites; V, vesicular structure found in serial sections to be continuous with the neural tube. A. A section of a series V sandwich, cultivated for 13 days. The operational scheme was the same as for Fig. 2 D. x 100. B. A section of a series VT sandwich, cultivated for 11 days. A strip of the neural plate of a stage 1412 embryo including presumptive tail spinal-cord and somite regions was explanted together with the tail notochord from the same embryo, x 100. C. Sandwich no. VII (10) of series VI had broken into fragments which were cultivated for 13 days. A section of each is shown. The operational scheme was the same as in B. x 35. D. A section of one of the two fragments of sandwich no. VlI(10) of series VII, cultivated for 13 days. Presumptive tail-somite region of the neural plate of a stage 1412 embryo together with its tail notochord was explanted, x 35. E. A high-power photomicrograph of part of the section shown in D. x 400.

Figure 3

Abbreviations:. N, neural tube; NC, notochord; S, somites; V, vesicular structure found in serial sections to be continuous with the neural tube. A. A section of a series V sandwich, cultivated for 13 days. The operational scheme was the same as for Fig. 2 D. x 100. B. A section of a series VT sandwich, cultivated for 11 days. A strip of the neural plate of a stage 1412 embryo including presumptive tail spinal-cord and somite regions was explanted together with the tail notochord from the same embryo, x 100. C. Sandwich no. VII (10) of series VI had broken into fragments which were cultivated for 13 days. A section of each is shown. The operational scheme was the same as in B. x 35. D. A section of one of the two fragments of sandwich no. VlI(10) of series VII, cultivated for 13 days. Presumptive tail-somite region of the neural plate of a stage 1412 embryo together with its tail notochord was explanted, x 35. E. A high-power photomicrograph of part of the section shown in D. x 400.

Almost no tail developed in ten out of thirteen donors of the presumptive tail tissues. In the remaining three the tail was small; one of these was sectioned and contained sparse musculature.

Series VII

In these experiments only the prospective tail somite region of the neural plate and a piece of underlying notochord were cut out from stage embryos and combined in a sandwich. The explants were cultivated for 12 days. Out of ten explants six were lost. Two contained neural tubes, notochord and somites and one explant had only neural tube with some mesenchyme and connective tissue. In this case also the notochordal element perhaps slipped out of the sandwich early during cultivation. The fourth surviving explant had broken into two fragments, one of which was lost during processing and the other found to contain coalescing groups of myoblasts close to the inner side of the epidermis. They appeared to be segmentally arranged (Fig. 3D, E).

With one exception all other donors of the explanted material lacked tails of any appreciable length. A normal tail had developed in one case.

The results of the experimental series I—III clearly point to the conclusion that the presumptive tail somite region of the neural plate in Ambystoma mexicanum at even late neurula stage is incapable of self-differentiation into muscle tissue if cultivated alone in an ectodermal sandwich. For Triton pyrrhogaster, Nakamura (1947) reported that whereas at early neurula stage the presumptive tail somite material is unable to form muscles it is determined enough to follow its prospective histological fate by the middle of neurulation and acquires the ability of stretching and segmentation at late neurula stage. Bijtel (1936, 1958) also obtained good muscle differentiation of the hind part of the neural plate when it was isolated from prominent neural fold (mid-neurula) stage of A. mexicanum embryos. It should be noted that both these authors transplanted the neural-plate strips into the flank or belly of host neurulae. This technique does not permit the development of the part concerned in complete isolation from possible inductive influences of the host tissues. Both Nakamura (1947) and Smithberg (1954) found that the ability to differentiate into muscle tissue is not present in the posterior region of the neural plate in early neurula and it is gradually acquired during later stages of neurulation. Our results support this conclusion but with the qualification that in isolation muscle differentiation capacities are not manifested even when the prospective tail somite material is taken from late neurula stage.

The results of series I also suggest the conclusion that in early neurula stage (no. 13) when neural plate is barely recognizable, the presumptive tail region of the plate possesses definite neural differentiation capacities which are manifested even when it is cultivated alone. Thus, neural tissue differentiated in nine out of fourteen surviving sandwiched explants of this series. These included seven cases of good neural-tube formation and two cases in which melanophores together with some mesenchyme were found. Melanophores are of neural origin and their differentiation would indicate a rather weak degree of neuralization. Hence, these two cases should also be regarded as indicating neural tendencies in the explanted material. However, it is possible that some prospective neural-crest material may have been inadvertently included in the strip cut out from the neural plate. At stage 13 neural folds are not present and the neural crest region is not demarcated clearly.

The neural differentiation tendencies in the caudal part of the neural plate are, however, sharply reduced, if not completely abolished, in the mid- and late neurula stages. In series II and III, in which the donors of this region of the neural plate were at stages and 17-18, respectively, only five out of twenty-one surviving explants showed neural differentiation and two of these contained only melanophores.

Eyal-Giladi (1954) analysed the differentiation tendencies of the presumptive neurectoderm of A. mexicanum at different gastrula stages as it came successively under the influence of cranio-caudal regions of the archenteric roof. It was found that these tendencies change from those of more cephalic to more caudal structures as the particular area of the neurectoderm comes under the influence of more caudal parts of the notochord. The posterior region of the neural plate comes to overlie the tail notochord at the very end of gastrulation and, therefore, inferring from the results of Eyal-Giladi, it can be expected that a change in differentiation tendencies would similarly occur in this region of the neural plate, and that this change will proceed to completion during post-gastrula stages. Accordingly, neural differentiation of isolated prospective tail somite region should be expected in early neurula stages. Loss of this tendency and mesodermal properties should be manifested in later stages, as these additional results confirm.

It is not essential that neuralization precede mesodermalization of the ectoderm. Spofford (1948) demonstrated this by transplanting un-induced, competent ectoderm over the tail notochord in Ambystoma neurulae, and Toivonen (1958) has shown the same in ectodermal sandwiches using guinea-pig bone marrow as the inductor. As far as production of purely mesodermal structures is concerned, the above is also accepted by Nieuwkoop (see footnote in Nieuwkoop & Grinten, 1961). In our opinion, although under experimental conditions competent ectoderm can be made to develop into mesodermal structures directly without passing through a phase of neuralization, this does not appear to occur during normal embryogenesis as far as the tail somite material is concerned. The particular dorsal and posterior position of this region of neurectoderm in the gastrula and the presence in it of neural competence together with the path of chordal invagination determine that it must pass through a phase of neuralization before acquiring properties for mesodermal differentiation. However, the neural determination of the caudal part of the neural plate remains labile and is ultimately abolished under the persistent inductive influence of the tail notochord during neurulation.

According to Nakamura (1947), the prospective tail somite portion of the neural plate of even an early neurula forms well-differentiated muscle if transplanted in combination with prospective tail spinal cord and/or notochord. Takaya’s (1959) results would also support these observations. In his explants of the neural plate, somites developed along with the spinal cord, even when no notochord appeared in the sandwiches. The results obtained in our experiments do not confirm these observations of Nakamura and Takaya made on other species. Thus, in our series V only neural tubes were formed with some undifferentiated mesenchyme in nine out of ten surviving explants although the explanted material included both tail somite and spinal-cord regions of the neural plate. It may be argued that in our experiments the isolated strip of the neural plate may not have included the tail somite region. This is, however, highly improbable because if this were so the donors of the neural-plate strip should have been expected to develop more or less normal tails as the loss of the spinalcord material at this stage is normally regulated to a great extent. This was so in the donors of our series IV, in which only prospective spinal-cord area was removed; the great majority of donors in this series developed almost normal tails. In series V, in which the removed neural-plate strip included both tail somite and spinal-cord regions, the great majority of donors developed either very small and deficient tails or no tails at all. However, neural-tube formations occurred in the majority of explants of this series as for the explants of series IV. It seems correct to conclude that the spinal cord did not support muscle differentiation from the presumptive somite material in the sandwiches of series V. In Nakamura’s experiments mesodermal inductive influences from the tissues of the hosts may have caused muscle differentiation in the transplanted prospective somite material of the neural plate in combination with spinal cord from donor neurulae. It is not possible to visualize such extraneous influences in Takaya’s experiments, at least for those cases in which somites developed along with spinal cord even in the absence of notochord.

Smithberg (1954) emphasized the importance of the notochord for proper differentiation of muscle tissue from the neural plate material in Rana pipiens embryos. The results obtained in our series VI and VII confirm and support Smithberg. In the former series the explanted material was a combination of prospective spinal-cord, tail somite region and tail notochord. In the latter series tail somite portion of the neural plate was combined with the notochord alone. In both series somite developed along with spinal cord and notochord in nine out of twelve cases. In one of the remaining three cases, belonging to series VI, notochord was not found but neural tube and somite development had occurred. Perhaps the notochord slipped out of the sandwich during cultivation but stayed long enough to induce muscle differentiation in the presumptive somite material. One of the other two explants, belonging to series VII, contained only neural tube and dense mesenchyme and the other had only a few small somites. Notochord was absent in both. It should be noted, however, that among all the series of the present investigation somite development occurred only in those explants in which notochord had been deliberately included. Yamada (1940) had found in Triturus that proximity of notochord was important for proper differentiation even of already invaginated trunk somitic mesoderm isolated from neurula stages and explanted. It should be even more important at stages when the prospective mesoderm is still on the surface and not yet completely determined.

It may be noted that in series VII, although the neural-plate strip cut out and combined with the notochord did not include spinal-cord material, the neural tube did appear in all four surviving explants. Smithberg (1954) found the same in his material. The notochordal piece included in the sandwich is not a pure mesodermal inductor and that may explain why both neural and mesodermal structures were formed.

  1. Differentiation capacities of the presumptive tail somite region of the neural plate in Ambystoma mexicanum embryos were investigated by culturing this region sandwiched in ectoderm either alone or in combination with spinal cord and/or notochord.

  2. When the presumptive tail somite material was taken from stages 13, or 17-18 embryos and cultured alone it did not differentiate into muscle.

  3. The prospective tail somite material from early stage 13 neurulae on being explanted alone gave rise to neural differentiations in nine out of fourteen surviving explants. The neural tendency was considerably reduced in material taken from stages and 17-18 neurulae and explanted similarly. Small neural formations occurred in only five out of twenty-one cases from these older stages.

  4. In explants of prospective tail somite material from stage embryos combined with spinal-cord rudiment of the same embryos, good neural tubes were formed in nine out of ten cases but muscle differentiation did not occur in any case.

  5. When explants contained a combination of tail somite and spinal-cord regions of the neural plate and tail notochord or of the somite material and notochord alone, taken from stage neurulae, somite development occurred in eleven out of twelve cases; the notochord and spinal cord were also present in nine of the eleven explants.

Capacité de différenciation de la région des somites caudaux présomptifs de la plaque neurale chez les embryons d’Ambystoma mexicanum

  1. On a étudié la capacité de différenciation de la région des somites caudaux présomptifs de la plaque neurale d’embryons d’Æ mexicanum en cultivant cette région, dans un sandwich d’ectoderme, soit isolée, soit en combinaison avec de la moelle épinière et (ou) de la notochorde.

  2. Quand le matériel présomptif des somites caudaux a été prélevé sur des embryons des stades 13, ou 17-18 et mis seul en sandwich, il ne s’est pas différencié en muscles, sauf dans un cas.

  3. Le matériel présomptif des somites caudaux de neurulas à partir du début du stade 13, explanté isolément, a donné naissance à des différenciations neurales dans neuf explants sur quatorze survivants. La tendance neurale était considérablement réduite dans ce matériel pris sur des neurulas des stades 14| et 17-18 et explanté de la même façon. De petites formations neurales sont apparues dans cinq cas seulement sur vingt et un de ces stades plus âgés.

  4. Dans les explants de matériel somitique caudal présomptif d’embryons du stade combiné à une ébauche de moelle épinière des mêmes embryons, des tubes nerveaux convenables se sont formés dans neuf cas sur dix mais la différenciation musculaire ne s’est jamais produite.

  5. Quand les explants renfermaient une combinaison formée de régions des somites caudaux et de la moelle épinière de la plaque neurale avec de la notochorde caudale, ou de matériel somitique et de notochorde seule, prélevés sur des neurales du stade, le développement des somites a eu lieu dans onze cas sur douze; la notochorde et la moelle épinière étaient également présentes dans neuf des onze explants.

I am grateful to Professor Dr P. D. Nieuwkoop, Director of Hubrecht Laboratory, Utrecht (Holland), who suggested the problem and provided the necessary facilities during my work at this laboratory with the 5th International Research Team in Embryology. The experimental work was done at Utrecht and the data were processed and analysed at the Zoology Department of the University of Rajasthan, Jaipur (India). I am also grateful to the government of the Netherlands for a Fellowship which made this study possible.

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