ORGANIC EVOLUTION 



171 



in a hollow sphere of cells (fig. 107 d) arranged around the 

 now distended wall of the old egg, and called a blastula. 

 Then with the continued increase of cells by fission the wall 

 of the blastula becomes pushed in on one side like a hollow 

 rubber ball indented with the thumb. As the central cavity 

 deepens the walls come closer together, and their convergent 

 edges form a round opening, the blastopore. In this two- 

 layered body we may already 

 recognize ectoderm and en- 

 doderm, having the same 

 general relations as in the 

 body of the hydra. These 

 are the primary germ layers. 

 Here they are not obviously 

 differentiated at first except 

 by their position. This form 

 of embryonic body is called a 

 gastrula, and the process of 

 invagination by which it is 

 produced is called gastrula- 

 tion. The cavity which cor- 

 responds to the food cavity 

 of the hydra is called the 

 arch-enter on. 



A third germ layer, the 

 mesoderm, appears in the 

 worm before gastrulation is 

 completed. It originates as 

 an ingrowth of cells into the 

 narrow segmentation cavity, 

 as indicated in the accom- 

 panying diagrams. The 



diagrams of figure 108 show also how it splits into two layers 

 (joined by rows of cells that later develop into the septa) the 



FIG. 108. Diagrams illustrating fur- 

 ther development of the earth 

 worm, a and b, longitudinal sections 

 of later stages ; c, d and e, cross sec- 

 tions of t ne body showing the 

 splitting of the mesoderm and the 

 formation of the ventral nerve cord. 

 The ectoderm is white; the endo- 

 derm is crosslined and the mesoderm 

 is hatcheled; n, nerve cord. 



