198 M. E. Cl'aparede on the Devetopment of Neritina fiuviatilis. 



spheres undergo segmentation, but they are not surrounded by 

 a peculiar membrane. There is no perceptible difference be- 

 tween them ; and in Neritina no such distinction in the mode 

 of segmentation as that discovered by Bask in Purpura y was to 

 be seen. The only difference is in their ultimate destiny; and 

 it appears probable that the genesis of both structures is the 

 same, as the segmentation of the ^'egg-like bodies" is in favour 

 of their being true eggs ; and the distinction observed by Busk 

 may not be of great importance, as it is asserted only that many 

 of the "egg-like bodies" contained in the capsule exhibit a very 

 marked irregularity in the first segmentation ; so that it is not 

 impossible that the others might have presented the same thing 

 less distinctly ; and, moreover, there is no certain proof that the 

 former were really the so-called true eggs. All the eggs in the 

 capsules both of Neritina and Purpura may therefore be regarded 

 as genuine eggs, but how most of them are arrested in their 

 development still remains a mystery. 



When the abortive spheres in the capsules of Purpura have 

 undergone segmentation, they exhibit a decided tendency to 

 become amalgamated, and adhere to each other with such tena- 

 city, that they cannot be separated without difficulty ; finally, 

 their boundary-lines disappear entirely, and there remains only 

 a uniform conglomeration of small vitelline segments. This 

 never occurs in Neritina, At the time when the embryo makes 

 its appearance, the abortive segmented eggs split up into groups 

 of globules, each of which is about half the size of an entire egg, 

 and sometimes even smaller. These usually consist of one large 

 and numerous smaller yelk-spherules. But there is never even 

 a slight mutual adhesion of these groups, and they remain sepa- 

 rate until they disappear by being devoured by the embryo. 



The observation of the different stages of development was 

 rendered difficult by the opacity of the capsules, which required 

 to be opened to enable their contents to be examined, and by 

 the fact that the Neritina do not lay their eggs in captivity, 

 probably because the still water does not suit them. The 

 smallest embryos detected, which had completely lost their ciliary 

 coat, formed an irregular cylinder, divided into two parts by a 

 circular notch; these parts may be distinguished as "cephalic" 

 and "abdominal," names employed by Vogt in Actaon. The 

 cephalic portion bears an elevation on its dorsal surface, forming 

 a more or less distinct oval swelling, beset with very delicate 

 cilia. This is the first trace of the velum. Close before it, at 

 the anterior end of the animal, is a shallow impression, — the 

 mouth. Immediately below this a disciform organ soon makes 

 its appearance, which is at first very narrow and short, but gra- 

 dually increases in size posteriorly. This is the first indication 



