Notes on the Development of Mollusca

—The eggs of Elysia viridis are extremely favorable for showing the earliest developmental phases, such as the male and female pronuclei (impregnation), constriction of the polar cells and the concurrent well-marked amoeboid movements of the egg, radial striæ and other phenomena of segmentation, all of which can be readily studied in the living ovum.

In the majority of the Nudibranchs the segmentation is very regular, resulting in the formation of a well-marked gastrula, which is usually formed by a kind of rolling over combined with invagination. The extreme of the former process of gastrula formation is apparently reached in the Nematoid Cucullanus¹ The slit-like blastopore closes over from behind forwards, and I have reason to believe that in Fiona (sp. ?) at least the blastopore either becomes the permanent mouth, or the latter is invaginated at the spot where the former finally closes up.

At the commencement of the Veliger-stage two large cilia make their appearance in the centre of the velar area.³ There can be no doubt that these cilia, which, owing to the difficulties of observation, have escaped previous investigators, are homologous with the cilia or ciliated tuft found in a similar spot of the pre-oral lobe in some Lamellibranchiata,¹ Dentalium,² Chiton,³ Heteropoda⁴ and Pteropoda,⁵ and which is of such constant occurrence amongst the Vermes. These cilia are retained till the velum itself is lost. I have observed them in Fiona, Polyaera quadrilineata, Elysia, and Philine aperla, and have no doubt that they are characteristic of the whole group.

The character of the groove at the edge of the velum and its relation to the mouth is well shown in figs. 4 and 5. The groove is clothed with fine cilia and the lower border has a fringe of larger cilia, but these are not nearly so large as the powerful motor cilia of the upper border. We thus have a pre- and a post-oral circle of cilia. This groove has been described and figured by Pol as occurring in Pteropods and Heteropods, and is indicated by Lankester in a sketch of the veliger of Polycera quadrilineata, though not described by him.⁶ Fol ascribes a nutritive function to this groove. Though I carefully looked for food particles passing along it, I was disappointed, but I have no doubt that such is the case. Balfour further alludes to it in vol. ii, p. 306, of his ‘Treatise’ The velar groove is probably common to all the Nudibranchs.¹

In all the Gasteropoda I have examined I have found a patch of cilia either round the anus, or at that spot where the anus will appear.

I have often seen, in optical section, thickenings of the epiblast in the regions of the supra-œsophageal and pedal ganglia, and have no doubt that they were the rudiments of the nervous system, but this can only be satisfactorily demonstrated by means of sections. The sense organs, too, have an epiblastic origin.¹

—The prevalence of westerly storms when I was at Naples gave me an opportunity of examining Janthina fragilis. The ovum, which is crowded with coarse yolk spherules, divides into four segmentation spheres in the ordinary manner. Fig. 6 shows the next stage, the four new epiblast cells being much smaller in size, and are formed of clear protoplasm. I am inclined to think that the next stage is formed by other four cells being segmented off from the four large yolk cells (fig. 8) ; be this as it may an epiblastic cap is soon formed, which spreads over the yolk cells. Fig. 10 shows that at the usual position, at the tip of the blastopore, mesoderm cells (m) are segmented off from the yolk cells.

The further development of Janthina presents us with nothing which is not common to most other Prosobranchs. I would, however, draw attention to the large violet mass which is seen on the right side of the embryo close to and dorsal to the anus. A similar pigmented mass is met with in many Opitho-branchs, large and red on the right side in Philine aperta, a small violet spot on the left side of Elysia virides ; in Pleuro-branchidium, as first noticed by Lankester (loc. cit.), there is one on each side coloured pink. It would be interesting to discover the meaning of this pigment.

It is well known that the nervous system arises from the epiblast throughout the animal kingdom, with the exception of certain nerve elements in the Cœlenterata, which have a hypoblastic origin, and of the central nervous system in Prosobranch, Gasteropoda, and in Cephalopoda, which is stated to arise from the mesoblast. It is quite comprehensible how the former exception to the general rule should arise, but the two latter are, to say the least of it, very anomalous ; and it was for the purpose of attempting to clear up the difficulty that the following researches were made.

In all the groups of the Gasteropoda, with the exception above noted, the origin of the nervous system from the epiblast has been observed with more or less accuracy. I shall now describe the manner in which the central nervous system develops in two genera of the Prosobranchiata.

In Purpura lapillus, at the stage represented in fig. 12, two large thickenings of the transparent skin are seen with the velum in close connection with the developing eyes and incipient tentacles, and two similar masses occur in the foot, having an intimate relationship with the otocysts. A transverse section through this region shows us that the nervous system is in process of development by proliferation from these paired thickenings of the epiblast. Fig. 14 gives a more highly magnified view of one of the supra-œsophageal ganglia thus being formed. It is worth while to compare this figure with that given by Rabl of Planorbis.¹ From observations made by transverse sections I have no hesitation in expressing my conviction that a precisely similar origin of the nervous system occurs in Murex erinaceus. Fig. 15 shows the proliferation (pg) which forms a pedal ganglion. Close by it is seen the newly-formed otocyst (ot). The sense organs are developed by involution from the same tissue (see figs. 16 and 17).

These statements differ most materially from those of Bobretzky,¹ who “plodded at section-cutting to elucidate the question as to the origin of the nerve elements,” and who arrived at the conclusion “that (in Fusus) the ganglia arise as a massing together of mesoderm cells” (p. 143). I would venture to suggest that we may regard his account as either depending upon the occurrence in Fusus of “precocious segregation” (Lankester), as is apparently the case in the Cephalopoda, or, more probably, arising from an error of observation on the part of this accurate and painstaking observer ; for it seems to be very unlikely that Fusus should differ so much in the development of these important organs from the allied genera Purpura and Murex.

According to Fol,² in Limax, the pedal ganglia arise from the mesoblast of the foot, while the supra-œsophageal ganglia are developed from the epiblast of the velum ; this manifestly requires corroboration.

Accepting these conclusions then, the Cephalopoda appear to be unique in having a mesoblastic nervous system—a point which requires to be again worked over.