ONTOGENY 61 



inner, the hypoblast or endoderm, the lining of the most 

 important parts of the food canal and its outgrowths is 

 formed. In all animals above sponges and coelenterates 

 a middle layer appears which is called the mesoblast or 



C 

 A. Section of Blastula. B. Section of Gastrula. C. Monila. 



mesoderm, and it gives rise in the adult to such structure 

 as muscles, skeleton, and connective tissue, gonads, &c. 



Now in those eggs in which the segmentation is only 

 partial, the segmentation is limited to a small super- 

 ficial area lying on the yolk, and this area is called the 

 blastoderm. When the blastoderm has reached a certain 

 stage, a modified form of blastula is formed by a differen- 

 tiation into superficial and deeper layers, and gastrula- 

 tion takes place not by a simple invagination, but by a 

 combination of folding and invagination. Yet the final 

 result of all the types is essentially the same, giving 

 rise alike to a three-layered embryo. 



In the higher animals this formation of a three- 



