218 



ZOOLOGY 



(Fig. 165, C), the final result being the production of an embryo 

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



Fic:. 104.— Segmentation of the oosperm ^-in Ctenophora. mg. megameres ; mi. inicromeres ; 

 plsm. protoplasm ; i/]c. yolk. (Modified from Korschelt and Heider.) 



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

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

 preceding types, the microrneres forming the ectoderm, the mega- 



A . B C 



Fir.. 165. — Three stages in the development of Ctejiopliora. ma. megameres ; mi. microrneres. 

 (From Lang's Comparatiee Anatomy.) 



meres the endoderm, and the ventral edge of the ectodermal 

 investment representing the blastopore. There is, however, no 

 archentcron or gastrula-cavity, and the stage has been produced, 



R n „e C 



Fig. lOG. — Tlirce stages in the development of CaUianira. d. infundibulum ; ec. ectoderm ; 

 tn. endoderm ; vie. mesoderm ; st. stoniud;tiUm. (From Lang's Comparative Anatomy.) 



not by a process of invagination or tucking-in, but by one of epiboly 

 or overgrowth. 



The endoderm-cells increase in number, and become much 

 elongated and arranged obliquely, their long axes radiating, 



