XII 



PHYLUM MOLLUSCA 



751 



entirely from cells that migrate inwards from the ectoderm and 

 come to fill the segmentation-cavity. The megameres themselves 

 eventually become converted into endoderm cells. A segmentation- 

 cavity is developed between the micromeres and the megameres, 

 and the result is the formation of the blast ula, 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 

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



7TLIC 



ties, mesoderm ; pol. polar bodies. (After Korschelt and Heider.) 



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

 cavity is of considerable size, an invagination takes place. 



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

 characteristic of the development of the Gastropoda, The former 

 is most typically developed in Patella', in other Gastropods it 

 undergoes more or less modification. In Patella (Fig. 644) there 

 is a ciliated blastula (A) which has on one side the large megameres 

 The latter become enclosed by the micromeres, and the foundation 



3 B 2 



