218 



ZOOLOGY 



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

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



FIG. 104. Segmentation of the oospsrm in Ctenophora. ing. megamcres ; mi. micromcrcs ; 

 pfsm. protoplasm ; yk. yolk. (Modified from Korschelt and Holder.) 



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- 



Fro. 165. Three stages in the development of Ctenophora. ma. megameres ; mi. micromeres. 

 (From Lang's Comparative Anatomy.) 



meres the endoderm, and the ventral edge of the ectodermal 

 investment representing the blastopore. There is, however, no 

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



B C 



FIG. 166. Three stages in the development of Callianira. d. infimdibulum ; ec. ectoderm ; 

 en. endoderm ; me. mcsoderm ; st. stomod%um. (From Lang's Comparative Anatomy.) 



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

 or overgrowth. 



The endoderm-cells increase in number, and become much 

 elongated and arranged obliquely, their long axes radiating, 



