24 



ZOOLOGY 



SECT. 



face so as to give it somewhat the appearance of the fruit of 

 the mulberry, whence it is termed the mulberry body or morula 

 stage. The blastomeres next become arranged regularly in a 

 single layer the embryo (Fig. 9, A) assuming the form of a hollow 

 sphere, the blastosphere or blastula, with a wall composed of a 



arc] i; 



ABC 



FIG. 9. Gastrulation. 



arch, archenteron ; bl. blastopore ; erto. ectoderm ; endo. endoderm. 



single layer of cells enclosing a cavity the segmentation cavity 



or blastoccele. 

 One side of the hollow blastula next becomes pushed inwards or 



invaginated (Fig. 9, B, C), as one might push in one side of a hollow 



india-rubber ball, the result of this process of invagination, or 



gastrulation as it is termed, being the 

 formation of a cup the gastmla (Fig. 10) 

 with a double wall. The cavity of the 

 cup-shaped gastrula is the archenteron or 

 primitive digestive cavity ; the opening is 

 termed the blastopore ; the outer layer of the 

 wall of the cup is the ectoderm (or epiblast), 

 the inner the endoderm (or liypoblast). The 

 ectoderm and endoderm are the primary 

 germinal layers of the embryo ; from one or 

 both of them are developed the cells of a 

 third layer the mesoderm (mesoblast) 

 which is subsequently formed between them. 

 This mode of formation of the primary 

 germinal layers in holoblastic oosperms by 

 a process of gastrulation prevails in a number 

 of different sections of the animal kingdom. 



In many animals, however, it becomes modified or disguised in 



various ways ; and in certain cases it is doubtful if there occurs 



anything of the nature of true gastrulation. 



The cells of the three germinal layers give rise to the various 



organs of the body of the fully-formed animal each layer having 



Fid. 10. Gastrula in 

 longitudinal section, a, 

 blastopore ; b, arch- 

 enteron ; c, endoderm ; 

 d, ectoderm. (From 

 Gegenbaur's Compara- 

 tive Anatomy.) 



