TV 



PHYLUM COELENTERATA 



215 



producing a sixteen-celled stage (E, F) in which we can distinguish 

 eight large, ventral, yolk-containing cells or megameres (mg.), 

 and eight small, dorsal, protoplasmic cells or micromeres (mi.). 



A 



k 



.jblsm 



D/ 



FIG. 165. Segmentation of the oosperm in C'tenophora. mg. megameres ; mi. micromeres 

 plsm. protoplasm ; yk. yolk. (Modified from Korschelt and Heider.) 



The micromeres increase rapidly in number by division, and are 

 further added to by new small cells being budded off from the 

 megameres (Fig. 165, G, H, and Fig. 166, A). The result of this 



FIG. 166. Three stages in the development of Ctenophora. ma. megameres ; mi. micromeres. 



(From Lang's Comparative Anatomy.) 



increase is that the micromeres gradually overspread the megameres 

 (Fig. 166, C}, the final result being the production of an embryo 

 consisting of a central mass of large yolk- containing cells (ma.), 



me 



me 



FIG. 167. Three stages in the development of Callianira. d. infundibulum ; ec. ectoderm ; 

 en. endoderm ; me. small cells divided off from endodenn ; st. stomodseum. (From 

 Lang's Comparative Anatomy.) 



partly surrounded by an epithelium-like layer, incomplete below, 

 of small cells (mi.). This stage corresponds with the gastrula of 



