700 



ZOOLOGY 



SECT. 



remarkably close resemblance to the process of segmentation of 

 the ovum of a Polyclad as described on p. 257. 



The megameres then give off internally four small endoderm 

 cells, and from one of these (endomesoderm cell) are formed two 

 primitive mesoderm cells, from which the cells of the mesoderm 

 are developed The megameres themselves eventually become 

 converted into endoderm cells. A segmentation-cavity is de- 

 veloped between the micromeres and the megameres, and the 



pol 



TTtlC 



Tries 



FIG. 606. Diagram of the segmentation and formation of the germinal layers of the Gastropoda, 

 A and B, lateral view ; C F, viewed from the animal pole ; H, from the vegetal pole ^ 

 &', in optical section: n-t. ectoderm; c/c'. endoderm; ;<,.<. mesoderm ; pol. polar bodies. 

 (After Korschelt and Heider.) 



result is the formation of the blastula, one side of which (vegetal 

 pole) is greatly thickened owing to its consisting of the large 

 megameres, the opposite side (animal pole) being made up of the 

 micromeres. This may become a gastrula by epiboly or over- 

 growth of the ectoderm over the megameres, or, if the segmentation- 

 cavity is of considerable size, an invagination takes place. 



The two larval stages, the trochosphere and the veliger, are 

 characteristic of the development of the Gastropoda. The former 



