ABSTRACT
Attention is drawn to the regular, though transient, presence of aberrant nerve fibres (fibres égarées) in three different regions of human embryos and foetuses.
One of these regions is the cartilaginous basisphenoid into which fibres from the abducens nerves pass. The other two regions are the epithelial cord, known as the organ of Chievitz, and, in male specimens, the epithelium of the fused paramesonephric ducts.
The possible causes for the atypical behaviour of the aberrant nerve fibres is discussed. It is tentatively suggested that the abducens nerve fibres may be attracted medially by some influence emanating from the cranial end of the notochord. For the aberrant fibres in the uterus masculinas and in Chievitz’s organ the neurotropic attraction is presumed to emanate from the constituent epithelia of the two structures.
INTRODUCTION
In the early years of this century debate concerning the development of nerve fibres became more intense. During the previous decade, following the developmental studies of His (1883, 1886) and the early embryological studies of Ramon y Cajal (1890), the neurone theory as proclaimed by Waldeyer in 1891 seemed assured of victory; but when, with Apáthy and Bethe, new technical developments diverted attention from the whole neurone to its apparent constituents, the neurofibrillae, the simple concept of the outgrowth of the nerve fibre became enmeshed in complexity. Methods for their impregnation with silver were soon elaborated (Bielschowsky, 1904; Ramon y Cajal, 1903), and Held (1907) affirmed that a network of neurofibrillae preceded the appearance of the definitive nerve process. This claim became associated with the much older views of Hensen (1864, 1876) that protoplasmic strands were the forerunners of the nerve fibres and constituted a ground plan for the later development of the peripheral nervous system.
In 1908 Ramon y Cajal described a variety of observations on neurogenesis in the chick embryo which were inconsistent with this theory. Among them were some instances of aberrant fibres within the spinal cord and medulla at 4 days of incubation. These axones égarées included examples which penetrated the central canal; some of these re-entered the cord at another level. Others at first grew outwards in the normal way towards the surface of the cord but before reaching it turned sharply in the opposite direction and ended in a giant growthcone pointing inwards. Ramon y Cajal concluded that these abnormalities showed that in general the growth-cone possessed ‘une liberté de mouvement et d’allure qui ne concilie pas facilement avec la supposition d’un système de liens interneuronaux … ni avec celle d’une charpente de voies préétablies par lesquelles les fibres nerveuses jeunes seraient obligées de marcher’.
In more recent years several papers have been concerned with developing nerve fibres which follow an abnormal course, both within neuroepithelia and in the peripheral nervous system. In the developing inner ear, Tschernjachiwsky (1929) found that within the utriculus of a 3-month human foetus there were fibres which entered the lumen and which for some way grew freely into the fluid within. Not always, however, do the fibres égarées lose their way in this manner. In a further study of the auditory complex of nerves in the human foetus, Shute (1951) found that while some neurites were of this kind, others could ‘reach their destination by an abnormal or devious route’. Nor are aberrant fibres necessarily rare abnormalities. Bremer (1921) found that in 90 per cent, of human embryos at the 18-mm. stage, transient and recurrent branches of the abducens nerve of no apparent morphological significance run caudally to a number of muscles, both branchial and somitic.
In the present paper the study of fibres égarées in the human foetus is continued both in further observations on the developing abducens nerve and also in a study of such fibres within two rudimentary epithelial structures which are innervated at an abnormally early stage. These particular observations continue an inquiry begun by Ramon y Cajal himself in 1919 in a study of the penetration of nerve fibres into epithelia, the detailed course of which, he found, includes a phase of exploratory and transient elements.
Aberrant branches of the Vlth cranial nerve in human embryos and foetuses
The presence of recurrent branches arising in the course of the cranially directed abducens nerve has been recorded by a number of investigators for a number of vertebrates. Bremer (1921) summarized the literature on such fibres and added a detailed account of his own findings in a number of human embryos. He states that the recurrent fibres are infrequently to be found in Acanthias, Heptanchus, and in the pig, and that they are absent in the chick, the lizard, the sheep, and the rabbit. According to Bremer they are most commonly present in Man, ‘where they appear in about 90 per cent, of embryos up to 18 mm. and less frequently up to 31 mm., since they may begin to degenerate at 15 mm.’ Bremer’s material included no sections of embryos in which the peripheral nervous system had been specifically stained. All our silver impregnated (de Castro, Ranson, Bodian) human embryos possess the recurrent fibres and they are occasionally found in the younger foetal stages; we can also record their presence in equivalently treated rabbit embryos though, in this species, they are much fewer in number than in human specimens.
The recurrent fibres pass caudally to end, for the most part, in the mesenchyme surrounding the ventro-lateral aspect of the hindbrain. In a number of the specimens, however, some of the recurrent fibres unite to form a distinct nerve bundle which passes to the region of the glossopharyngeal nerve, at the level of its superior ganglion, with which it may unite; in two specimens the aberrant abducens fibres could be traced beyond the IXth nerve to join the vagus and accessory trunks. So far as we can determine these recurrent fibres disappear completely in later human development. When our findings are taken in conjunction with Bremer’s account, however, it is apparent that in a number of vertebrates there is a transient distribution of nerve fibres, which make their exit from the central nervous system in association with the Vlth nerve to regions of the embryo which cannot readily be considered as morphologically appropriate for this nerve.
There is another aberrant distribution of Vlth-nerve fibres which Bremer and, so far as we can determine, other investigators have missed, probably because silver impregnation techniques were not used in the preparation of the embryonic material studied. When the Vlth nerves are followed towards the primordia of the eye muscles in adequately impregnated human embryos of 15−20 mm. C.R. length, a number of fibres are seen to leave the main trunks of the nerves and to pass towards the anterior extremity of the notochord. When cartilage commences to appear in this region these medially directed aberrant fibres are caught up in the chondrification process and come to be included within the developing matrix of the basisphenoid cartilages (Plate 1, figs. 1, 2). There they can be found, like flies in amber, until about the 60-mm. stage, after which they apparently disappear; at any rate, they cannot be identified in our older foetuses, though the impregnation of the cranial nerves is generally very good.
In the earlier stages, as they are being caught up in the process of chondrification, the aberrant Vlth-nerve fibres provide a remarkable picture (Plate 1, figs. 3, 4). They are usually heavily impregnated with reduced silver and the individu?! fibres present an appearance of wire-like rigidity. No Schwann cells can be found on them. A single fibre frequently possesses complex angulations in its course, the microscopic picture presumably reflecting the constrictive influence on the fibre of the developing cartilage matrix. In such stages the endings of the aberrant fibres in the cartilage can frequently be identified by the possession of terminal bulbs. As development proceeds these bulbs disappear and the aberrant fibres end in an indefinite network in which the actual terminations can only dubiously be identified. Finally, in those stages (48-mm. and 62-mm. foetuses; Plate 1, fig. 5) at which the aberrant fibres can last be identified, the fibres would seem to have fragmented into separate lengths, for it is often impossible to trace continuity between fibres in the same or in adjacent sections. In yet older specimens no trace of the aberrant fibres in the basisphenoid region is to be found.
The distribution of the aberrant fibres to the basisphenoid region is, therefore, a transient one. Nevertheless, they are, it would seem, constantly present from the 12-mm. stage until the 60-mm. stage, that is to say, from about the 38th to the 90th day of development. Why they should grow into the mesodermal tissue surrounding the cranial extremity of the notochord is not apparent. Hughes & Tschumi (1960), however, have observed in grafting experiments in Xenopus what may be an attractive influence of notochord on motor nerve fibres. Whatever the explanation for the anomalous course and termination of the aberrant fibres, they become caught up in the chondrification of the basisphenoid. In this fact may lie the explanation for their persistence. No macrophages have been observed along the course of the disappearing fibres. The inability of such scavenging cells to enter the cartilage matrix could explain the anatomical persistence of the aberrant fibres which may, in fact, have ceased to be living elements for some time before they disappear. We are, of course, unable to comment effectively on the behaviour of the abducens nerve-cells which give origin to the aberrant fibres. They may degenerate with the disappearance of the fibres, or the latter themselves may merely be, or become, collateral branches of nerve fibres which proceed forward to aid in the innervation of the lateral rectus muscle.
Nerve fibres in Chievitz’s organ
In his account of the development of the salivary glands Chievitz (1885) described an epithelial strand which, in the 10-week human foetus, extends backwards in the mesoderm, parallel to the lateral border of the buccal sulcus, on the medial side of the mandible, and in close relation to the medial pterygoid muscle. In the intervening period attention has been drawn to the structure by a number of other investigators who have used such different names for it as the ramus mandibularis ductus parotidei (Weishaupt, 1911), the orbital inclusion (Schulte, 1913), Chievitz s organ (Broman, 1916), and the tractus buccopharyngeus (Brachet, 1919). It has been described in embryos of a number of mammalian species; Fahrenholz (1937) indicates that it has been found in Man, dog, horse, pig, cow, sheep, musk-deer, fallow-deer, hedgehog, mole, Tupaia, mouse, rat, guinea-pig, rabbit, Leptonychotes, Vespertilio, and opossum. It is, however, possible that different epithelial structures have been included within the descriptions, whatever general term is used for the strand. Further, the precise nature and significance of the epithelial band has by no means been established. It has been variously interpreted as a vestigial salivary gland (Broman, 1916), an organ of internal secretion (Schulte, 1913), a structure of undetermined nature (Moral, 1913), or as merely an epithelial strand of no morphological or functional import, which becomes separated from the cheek in the course of the developmental mechanical changes in the region. The last interpretation, which was put forward by Brachet (1919) and supported by Bollea (1924) and Casarotto (1928), is the one which seems currently to be the most acceptable and to which our own observations incline us.
Our present concern, however, is not to discuss the nature and significance of Chievitz’s organ itself but to draw attention to the remarkable association it possesses with branches of the buccal branch of the mandibular ramus of the trigeminal nerve. The association was briefly commented on by Brachet (1919); it is well shown in a 43-mm. human embryo (Plate 2, figs. 6, 7, 8) impregnated with de Castro’s modification of Cajal’s silver impregnation technique. In this specimen Chievitz’s organ has lost its connexion with the buccal epithelium, and it extends backwards on the medial side of the mandible as an elongated cord of compactly arranged epithelial cells. The buccal branch of the mandibular nerve can be traced in its course to the cheek lateral to the epithelial strand. As the branch passes the strand, a bundle of nerve fibres leaves it and passes along the surface of the epithelial cells. Individual fibres enter the strand and terminate in the interstices between its constituent cells. At this time, it must be stressed, the adjacent buccal epithelium possesses no innervation (Plate 2, fig-9).
The nerve fibres which enter Chievitz’s organ do not seem to possess neurilemmal sheaths. They are, of course, non-myelinated. Their method of termination is variable, some of them ending by becoming increasingly attenuated until they can no longer be resolved with light microscopy. Other fibres, however, possess definite end-bulbs. The richness of the nerve supply is quite remarkable, for it exceeds considerably that which is ever found, with the same impregnation techniques, in the epithelium of the buccal mucosa or of the cheek skin. Further, the innervation of Chievitz’s organ is of quite a different order from that of the salivary glands, in the epithelium of which no nerve fibres have been observed in any of our foetal specimens.
Our material includes a number of embryos and foetuses from the 15-mm. to the 150-mm. C.R. length stages. A definite innervation of the epithelium of Chievitz’s organ is first obvious in a 30-mm. embryo (Plate 3, fig. 10), and it appears to reach its maximum between the 40-mm. and 60-mm. stages. The nerve supply to the organ is distinctly reduced in a 92-mm. foetus in which a specific nerve bundle from the buccal branch of the mandibular trunk could no longer be identified. By the 150-mm. stage the number of nerve fibres in the organ has been very considerably reduced, and terminal bulbs can no longer be identified. A plexiform arrangement of nerve fibres round the epithelial cells is, however, still present (Plate 3, fig. 11). In none of our human material beyond the 150-mm. stage have we been able to identify Chievitz’s organ; this material, however, is not available in uninterrupted series. Its presence in a full-time human foetus has been recorded by Ramsey (1935).
Nerve fibres in relation to the paramesonephric ducts in male human foetuses
Figs. 12 and 13 of Plate 3 are photomicrographs of sections through the urogenital cords of a male and a female human foetus. Each specimen had a C.R. length of 46 mm.; their general embryological state and the associated obstetrical histories also suggest that the two specimens are of equivalent developmental age. The sections illustrated show, however, that the genital ducts are differentiating; in the male the paramesonephric (Mullerian) ducts have fused to form an irregular epithelial cord, while in the female they form a wellestablished utero-vaginal canal. On the other hand, the mesonephric (Wolffian) ducts are well developed in the male and are commencing to retrogress in the female. In each sex the genital ducts are surrounded by mesodermal condensations which in the female will develop into the myometrium and in the male into the fibrous and muscular tissue of the prostate gland. Flanking the mesodermal condensations are the differentiating neuroblasts of the pelvic ganglia. In each sex, in appropriately impregnated material, nerve fibres can be traced from these ganglia into the mesodermal condensations. In the female, in later foetal stages, these nerve fibres become distributed to the muscular coats of the uterus and upper vagina. In male foetuses, however, a remarkable difference is found. Some of the nerve fibres are indeed distributed to the myoblasts which will become part of the sheath of the prostate gland. But many of them pass to the wall of the epithelial tube—the uterus masculinus—arising from the conjoined paramesonephric ducts. Some of these fibres terminate on or in relation to the surface of the epithelium; others enter the epithelium (Plate 3, fig. 14; Plate 4, figs. 16, 18), and in older specimens bundles of the nerve fibres may actually perforate through into the lumen, which develops in the uterus masculinus, and pass upwards and downwards inside this cavity (Plate 4, figs. 15, 17). The nerve fibres frequently possess bulbous terminals (Plate 4, figs. 18, 19) and in the older specimens considerable lengths of nerve fibre may show a remarkably swollen appearance (Plate 4, fig. 19). We have found this invasion by nerve fibres of the epithelium of the uterus masculinus in human foetuses of as late as the 150-mm. C.R. length stage. Our older male foetal material, which is sparse and not very satisfactory, does not show such nerve fibres, and the aberrant innervation does not appear to be present in postnatal specimens. In the period of development between the 40-mm. C.R. stage and the 150-mm. C.R. stage, however, a rich distribution of nerve fibres to the epithelium of the uterus masculinus is present in all of our available specimens. Such appearances are not presented by any of our female foetuses and, we can add, no equivalent appearance has been found in relation to the retrogressing mesonephric ducts in female specimens.
DISCUSSION
Three examples of atypical behaviour by nerve fibres in the course of human development have been described. Each of them raises the question of why the nerve fibres have taken their aberrant courses. We have tentatively suggested that for the abducens fibres the attraction may be exercised by the cranial end of the notochord. The long course of this nerve, its proximity to the notochord in this course, and the relative remoteness of the somitic material which it has to reach, may be the explanation of the limitation of the aberrant process to this nerve. For the aberrant fibres in the uterus masculinus and in Chievitz’s organ the attraction presumably emanates from the epithelia of the two structures.
It was in his paper on the development of the vertebrate retina that Ramon y Cajal (1893) first advanced his hypothesis of ‘neurotropism’ in an attempt to account for the growth of a nerve fibre towards its destination, though he admitted the unlikelihood of any chemotropic stimulus influencing an elongating neurite across more than a relatively short distance. In the development of cutaneous nerves, however, the orientation of the fibres within the embryonic dermis and their manner of advance towards the overlying ectoderm appeared to him to provide clear instances of chemotropic attraction (Ramon y Cajal, 1919). He suggested that the embryonic dermis receives from the epithelium some ‘enzyme’ with a stimulatory action on the growth-cones of the advancing neurites.
To his description of the innervation of epithelia in late foetal and early post-natal life we can now add instances of the precocious entry of nerve fibres into two transitory organs belonging to different germ layers, at a time well before their period of dissolution, and when no adjacent epithelia have yet received any innervation. Whatever may be the factors which govern the entry of nerve fibres into epithelia in general, they must also operate, though prematurely, in these special instances.
Ramon y Cajal described four successive phases in the pattern of the invading nerve fibres in normal epithelia. In these prematurely innervated structures this sequence is apparently halted at the stage of irregular and exploratory neurites. In the uterus masculinus they become grossly enlarged and recall the fibres égarées which ended in giant end-bulbs described by Ramon y Cajal (1908) in the spinal cord of an embryo chick. The hypertrophy of the neurites within the uterus masculinus extends, however, to the whole terminal portion of the fibre, and is on a much larger scale.
Among his many observations of the growing nerves in the tail fins of tadpoles, Speidel (1933) described how the growth-cone of a fibre halted by an obstruction became enlarged. In the present instance, however, the undifferentiated epithelium of the uterus masculinus shows no features which might block the pathway of a growing fibre. Moreover, we see a wholly different reaction of neurites to mechanical constriction in the fibres of the abducens nerve trapped in the chondrifying basisphenoid; these take on a wire-like form with straight sections joined by sharp angles.
Heidenhain (1911) discussed the successive factors which may influence the gradual development of the peripheral nervous system and suggested that each organ receives an excess of nerve fibres, and that the final pattern of innervation is partly attained by the atrophy of superfluous elements. This must apply with especial force to organs in which the course of development is different in the two sexes. It seems reasonable to suppose that the paramesonephric duct in all embryos receives a total supply of neuroplasm adequate for the development of the basic pattern of innervation of a foetal uterus, but which in the male is excessive for the requirements of a developing prostatic utricle. In this way superfluous fibres may become engorged and break through the lumen of the organ with giant end-bulbs, as has here been described and illustrated.
Whatever the nature of the forces which govern the innervation of embryonic structures, it is clear that entering fibres are not merely passively accepted, but their growth and arrangement must be subject to a continuous control exercised in some way by the developing organ itself. Ramon y Cajal’s neurotropism still remains the most plausible explanation of these influences, particularly since the demonstration by Hamburger and his colleagues (Hamburger, 1952) that the growth of nerve fibres is stimulated by specific substances. Although these experiments still lie outside the course of normal embryonic development and no directional effects have yet been discovered, there is now proof that nerve fibres can respond to chemical influences (Levi-Montalcini, 1958). It would be an interesting inquiry in foetal endocrinology to examine whether the nerve fibres of developing reproductive organs are in any way sensitive to the action of sexual hormones.
RÉSUMÉ
Sur des fibres nerveuses aberrantes dans le développement de Vhomme
Les auteurs attirent l’attention sur la présence régulière, quoique passagère, de fibres nerveuses ‘égarées’ dans trois régions différentes des embryons et des fœtus humains.
Une de ces régions est le basisphénoïde cartilagineux dans lequel des fibres s’introduisent en provenance des nerfs efférents. Les deux autres régions sont le cordon épithélial, connu sous le nom d’organe de Chievitz, et, dans les spécimens mâles, l’épithélium des canaux paramésonéphriques fusionnés.
La cause possible du comportement atypique des fibres nerveuses égarées est discutée. On envisage l’hypothèse que les fibres nerveuses efférentes puissent être attirées en direction médiale par quelque influence émanée de l’extrémité craniale de la notochorde. Quant aux fibres aberrantes de l’utérus masculinus et de l’organe de Chievitz, elles pourraient être expliquées par l’attraction neurotropique qui émane des épithéliums constitutifs de ces deux structures.
REFERENCES
EXPLANATION OF PLATES
Fig. 1. Left abducens nerve (VI) and fibres from it which can be traced into the chondrifying basisphenoid in a 30-mm. human embryo (H. 180, de Castro technique). × 108.
Fig. 2. High-power view of the nerve fibres in the basisphenoid shown in fig. 1. × 684.
Fig. 3. Nerve fibres in left side of basisphenoid condensation in a 17·5-mm. human embryo (H. 191, de Castro technique). × 684.
Fig. 4. Nerve fibres in right side of basisphenoid condensation in 175-mm. human embryo (H. 191). × 684.
Fig. 5. Nerve fibres in basisphenoid cartilage in a 62-mm. human embryo (H. 209, de Castro technique). This is the oldest specimen in which such fibres have been found. × 684.
Fig. 1. Left abducens nerve (VI) and fibres from it which can be traced into the chondrifying basisphenoid in a 30-mm. human embryo (H. 180, de Castro technique). × 108.
Fig. 2. High-power view of the nerve fibres in the basisphenoid shown in fig. 1. × 684.
Fig. 3. Nerve fibres in left side of basisphenoid condensation in a 17·5-mm. human embryo (H. 191, de Castro technique). × 684.
Fig. 4. Nerve fibres in right side of basisphenoid condensation in 175-mm. human embryo (H. 191). × 684.
Fig. 5. Nerve fibres in basisphenoid cartilage in a 62-mm. human embryo (H. 209, de Castro technique). This is the oldest specimen in which such fibres have been found. × 684.
Fig. 6. Sagittal section through right Chievitz’s organ (C) in a 43-mm. human foetus (H. 206, de Castro technique) to show branches of the long buccal nerve (N) passing to it. × 108.
Fig. 7. High-power view of portion of fig. 6 to show nerve bundle skirting Chievitz’s organ and sending fibres amongst its epithelial cells. × 684.
Fig. 8. Nerve fibres in epithelium of left Chievitz’s organ in 43-mm. human foetus (H. 206, de Castro technique). × 684.
Fig. 9. Transverse section through right Chievitz’s organ (C) in a 30-mm. human embryo (H. 180, de Castro technique). Note nerve fibres entering epithelium of the organ and absence of such fibres in the parotid gland (P) and in the buccal epithelium (B). × 108.
Fig. 6. Sagittal section through right Chievitz’s organ (C) in a 43-mm. human foetus (H. 206, de Castro technique) to show branches of the long buccal nerve (N) passing to it. × 108.
Fig. 7. High-power view of portion of fig. 6 to show nerve bundle skirting Chievitz’s organ and sending fibres amongst its epithelial cells. × 684.
Fig. 8. Nerve fibres in epithelium of left Chievitz’s organ in 43-mm. human foetus (H. 206, de Castro technique). × 684.
Fig. 9. Transverse section through right Chievitz’s organ (C) in a 30-mm. human embryo (H. 180, de Castro technique). Note nerve fibres entering epithelium of the organ and absence of such fibres in the parotid gland (P) and in the buccal epithelium (B). × 108.
Fig. 10. High-power view of section through right Chievitz’s organ shown in Plate 2, fig. 9. x684.
Fig. 11. High-power view of section through right Chievitz’s organ in a 150-mm. human foetus (H. 177, de Castro technique). Nerve fibres are still present round the organ but are absent in the epithelium itself. × 684.
Fig. 12. Section of urogenital cord in a 46-mm. C.R. length male human foetus (H. 653). The fused and retrogressing paramesonephric ducts (p) appear as an epithelial cord in the centre of the microphotograph. On either side of this cord the well-formed mesonephric ducts (m) can be seen. × 108.
Fig. 13. Section of urogenital cord in a 46-mm. C.R. length female human foetus (H. 679). The fused paramesonephric ducts (p) constitute the central uterovaginal canal, on either side-of which can be seen the mesonephric ducts (m). The epithelium of these ducts is separated from the underlying connective tissue and is showing early retrogressive changes. The mesodermal condensation round the ducts is much more extensive and denser than in the male specimen of the same developmental stage illustrated in fig. 12. × 108.
Fig. 14. Longitudinal section through uterus masculinus (p) in a 92-mm. C.R. length male human foetus (H. 125, de Castro technique). Note large nerve fibre in epithelium of uterus masculinus and absence of such fibres in the basa deferentia (m). × 108.
Fig. 10. High-power view of section through right Chievitz’s organ shown in Plate 2, fig. 9. x684.
Fig. 11. High-power view of section through right Chievitz’s organ in a 150-mm. human foetus (H. 177, de Castro technique). Nerve fibres are still present round the organ but are absent in the epithelium itself. × 684.
Fig. 12. Section of urogenital cord in a 46-mm. C.R. length male human foetus (H. 653). The fused and retrogressing paramesonephric ducts (p) appear as an epithelial cord in the centre of the microphotograph. On either side of this cord the well-formed mesonephric ducts (m) can be seen. × 108.
Fig. 13. Section of urogenital cord in a 46-mm. C.R. length female human foetus (H. 679). The fused paramesonephric ducts (p) constitute the central uterovaginal canal, on either side-of which can be seen the mesonephric ducts (m). The epithelium of these ducts is separated from the underlying connective tissue and is showing early retrogressive changes. The mesodermal condensation round the ducts is much more extensive and denser than in the male specimen of the same developmental stage illustrated in fig. 12. × 108.
Fig. 14. Longitudinal section through uterus masculinus (p) in a 92-mm. C.R. length male human foetus (H. 125, de Castro technique). Note large nerve fibre in epithelium of uterus masculinus and absence of such fibres in the basa deferentia (m). × 108.
Fig. 15. Transverse section through urethra of an 84-mm. C.R. length male human foetus (H. 178, de Castro technique). Note bundle of nerve fibres passing through epithelium of uterus masculinus (p) to enter its lumen. Such fibres are absent from the common ejaculatory ducts (m), from the developing prostatic tubules, and from the epithelium of the urethra itself. × 81.
Fig. 16. Transverse section through urethra of same foetus illustrated in fig. 15, but at a more caudal level. Here the uterus masculinus (p) is a solid epithelial rod with numerous nerve fibres. Such fibres are absent from adjacent epithelium, including that of the common ejaculatory ducts (m). × 81.
Fig. 17. High-power view of uterus masculinus in fig. 15. The bundle of nerve fibres can be seen passing into lumen of the vestigial organ. × 513.
Fig. 18. High-power view of uterus masculinus in fig. 16. Note the numerous end-bulbs. × 513.
Fig. 19. High-power view of uterus masculinus in fig. 14 to show the terminal ramification of a swollen nerve fibre amongst the epithelial cells. × 513.
Fig. 15. Transverse section through urethra of an 84-mm. C.R. length male human foetus (H. 178, de Castro technique). Note bundle of nerve fibres passing through epithelium of uterus masculinus (p) to enter its lumen. Such fibres are absent from the common ejaculatory ducts (m), from the developing prostatic tubules, and from the epithelium of the urethra itself. × 81.
Fig. 16. Transverse section through urethra of same foetus illustrated in fig. 15, but at a more caudal level. Here the uterus masculinus (p) is a solid epithelial rod with numerous nerve fibres. Such fibres are absent from adjacent epithelium, including that of the common ejaculatory ducts (m). × 81.
Fig. 17. High-power view of uterus masculinus in fig. 15. The bundle of nerve fibres can be seen passing into lumen of the vestigial organ. × 513.
Fig. 18. High-power view of uterus masculinus in fig. 16. Note the numerous end-bulbs. × 513.
Fig. 19. High-power view of uterus masculinus in fig. 14 to show the terminal ramification of a swollen nerve fibre amongst the epithelial cells. × 513.