Miscellaneous. 77 



capsule, undergo segmentation, coming to maturity. In Fasciolaria 

 six or eight eggs develop in each capsule, the remaining ova show- 

 ing not the slightest traces of segmentation, the polar globules even 

 remaining unformed, although the ova contain a nucleus and a certain 

 amount of protoplasm and are not simply yolk-tnasses. 



The second portion of the forthcoming paper will deal with the 

 segmentation of Fiih/ur. The eggs are very large, containing much 

 yolk. A. single large polar globule is formed which contains some 

 yolk-granules. The ovum then segments into two and then four 

 equal spherules, and from these are separated four small protoplasmic 

 spherules, the micromeres. These then divide, after four more micro- 

 meres have been separated from the macromeres, as in the normal 

 Gasteropod segmentation. In one point, however, Fuhjur differs 

 from other forms which have been studied ; the number of genera- 

 tions of micromeres which are separated off from the macromeres is 

 very large — apparently they continue to be separated off as long as 

 any portion of the macromeres remains uncovered by the ectoderm. 

 And even after the blastopore has formed and closed at the nutritive 

 pole of the egg there can be seen, in the interior of the yolk-mass, 

 which represents the fused macromeres, or beneath the ectoderm at 

 the surface of the yolk-mass, cells which resemble, in certain charac- 

 teristic features, the micromeres which were separated from the 

 macromeres. These late-appearing micromeres, as they may be 

 termed, I believe, assist in tlio formation of the mesoderm, this 

 layer not being formed in its entirety from the primitive mesoderm- 

 cell. 



When the segmentation has progressed somewhat, but while the 

 micromeres are still confined to the formative pole, three of the 

 macromeres show elongated elevations upon their surfaces. The fourth 

 macromere has no elevation, but gives rise to the primitive mesoderm 

 cell. What the significance of the elevations may be I cannot 

 imagine, but there can bo no doubt that their appearance is 

 normal, and coincides with the formation of the first mesoderm-cell ; 

 this lies below the margin of the ectoderm-cell, and corresponds 

 exactly with the primitive mesoderm-cell of Nassa. 



At a later stage an invagination of the ectoderm at the formative 

 pole takes place. A deep depression is formed, which, however, 

 later disappears and leaves no trace. It apparently corresponds with 

 the similar invagination described by BlochmatJn in Neritina and by 

 Sarasin in Bithynia, though the description given in this latter case 

 is not very clear. 



The development of the endoderm I was not successful in observ- 

 ing. The blastopore is formed at the formative pole of the ovum, 

 and closes, the mouth being formed at the point of closure by an 

 ectodermal invagination which also gives rise to the oesophagus. 



Th(3 general considerations derived from the study of the segmen- 

 tation of the Gastcropods will be arranged under three divisions. 

 The first will treat of the influence of the yolk on the formation of 

 polar globules, the second on the phylogenetic significance of seg- 

 mentation, in which it will be held that the mode of segmentation 



