THE SEGMENTATION OF THE OVUM. 



83 



A very peculiar modification of the ordinary Gasteropod segmentation is 

 that described by Bobretzky for Nassa mutabilis 1 . 



The ovum contains a large amount of food yolk, and the protoplasm is 

 aggregated at the formative pole, adjoining which are placed the polar 

 bodies. An equatorial and a vertical furrow (fig. 43 A), the former near 

 the upper pole, appear simultaneously, and divide the ovum into three 

 segments, two small, each with a protoplasmic pole, and one large 

 entirely formed of volk material. One of the two small segments next com- 

 pletely fuses with the large segment (fig. 43 B), and after the fusion is com- 



A B 



Fin. 43. SEGMENTATION OF NASSA. MUTABILIS. (from Bobretzky). A. Upper half divided 



into two segments. B. One of these has fused with the large lower segment. 

 C. Four small and one large segment, one of the former fusing with the large 

 segment. D. Each of the four segments has given rise to a small segment. E. Small 

 segments have increased to thirty-six. 



plete, a triple segmentation of the large segment takes place as at the first 

 division, and at the same time the single small segment divides into two. hi 

 this way four partially protoplasmic segments and one yolk segment are 

 formed (fig. 43 C). One of the small segments again fuses with the large 

 segment, so that the number of segments becomes again reduced to four, 

 three small and one large. The protoplasmic ends of these segments are turned 

 towards each other, and where they meet four very small cells become budded 

 off, one from each segment (fig. 43 D). Four small cells are again budded 

 off twice in succession, while the original small cells remain passive, so that 

 there come to be twelve small and four large cells. In later stages the four 

 first-formed small cells give rise to still smaller cells and then the next- 

 formed do the same. The large cells continue also to give rise to small 

 ones, and finally, by a continuous process of division, and fresh budding of 

 small cells from large cells, a cap of small cells becomes formed covering 

 the four large cells which have in the meantime pressed themselves together 

 (fig. 43 E). A segmentation cavity of not inconsiderable dimensions 

 becomes established between this cap of small cells and the large cells. 



Arrhir f. mikr. An/it. Vol. xm. 1877. 



62 



