ORGANIC EVOLUTION 



171 



in a hollow sphere of cells (fig. loj 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- 

 Uon. 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 



Fig. 108. Diagrams illustrating fur- 1 c 1 , • 



ther development of the earth WOrm beiOrC gaStrulatlOn IS 



worm, a and 6, longitudinal sections 1-1 t, • • , 



of later stages; c, d and e, cross sec- Completed. it Originates aS 



tions of the loody showing the . , , ^ n • j j 1 



splitting of the mesoderm and the an lUgrOWth OI CCllS lUtO the 



formation of the ventral nerve cord. , , . 



The ectoderm is white, the endo- narrow Segmentation cavity, 



derm is crosslmed and the mesoderm • 1 • j 1 • ,^ 



is hatcheled.n, nerve cord. aS mdlCated lU thC aCCOm- 



panying diagrams. The 

 diagrams of figure io8 show also how it splits into two layers 

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



