Development of the Pectinibranchiata, 868 



greatly from the rule which we have given for the developDient 

 of Buccinum undatum ; and, on the other hand, prove that a 

 single &^^ cannot furnish sufficient materials for the future per- 

 fection of the animal. 



We have observed that in each egg-capsule there were one or 

 more eggs which, not being included in the act of conglomera- 

 tion, passed into a separate development. Even before the mass 

 of eggs is perfectly conglomerated, we see some which undergo 

 a segmentation, which, however, is subjected to many changes. 

 Thus the vitellus divides into 2 equal and opake spheres, each 

 of which again divides into 2 other equal spheres, so that we 

 then distinguish 4 equal spheres ; each of these divides in the 

 same way into 2 equal spheres, and this division continues until 

 the vitellus resembles a mulberry. But it is not rare for the 

 process of segmentation to stop at the formation of the first 2 

 spheres of segmentation, which begin to exude a clear humour, 

 and this, almost at the moment of exudation, changes into a de- 

 licate membrane (PI. XVII. fig. 6). As the exudation increases, 

 the membrane enlarges, and by degrees its upper surface is 

 covered with cilia, and soon with cirrhi. The embryo thus 

 formed begins to turn upon itself even in the fluid (fig. 7). 

 This, however, is not the usual way in which the formation of 

 embryos takes place in isolated eggs, for it very often happens 

 that the exudation and the development of the embryo do not 

 commence until the segmentation is well advanced, and after the 

 formation of 4, 8, or even 16 spheres of segmentation (figs. 8-12). 



Whether the segmentation goes through all its stages, or stops 

 at the first, it is certain that when the above-mentioned exuda- 

 tion has commenced, the segmentation ceases, and the formation 

 of the organs begins. Before entangling ourselves more deeply 

 with the development of these embryos, we must compare our 

 observations on those eggs of Buccinum undatum which undergo 

 segmentation, with those of other naturalists upon the process 

 of segmentation in difierent Mollusca. 



C. Vogt has observed, that when the first 4 spheres of seg- 

 mentation are formed, there are produced between them 4 other 

 small spheres, which set themselves in action and form the rudi- 

 ment of the peripheric organs, whilst the central spheres remain 

 longer without alteration. He thinks that these 4 small spheres 

 of segmentation may be formed by exudation from the larger 

 spheres. Leuckart has made the same observations upon the 

 development of the Heteropoda. J. Miiller has observed that 

 the germinal vesicle in Entoconcha mirabilis does not disappear, 

 but divides, and forms the clear bodies in the spheres of seg- 

 mentation. At first 2, and then 4 large spheres of segmentation 

 are formed ; they are opake, and each of them is furnished with 



