THE OASTEULA STAGE. 



53 



5. The Gastrula. 



On the completion of the blastula, as described above, the 

 multiplication of the cells ceases for a time, and the embryo 

 undergoes a great change in shape, whereby it becomes converted 

 into the form to which the name gastrula is given. This change 

 is brought about as follows. 



The lower surface of the blastula, consisting of the larger 

 cells, becomes flattened (Fig. 15, h), and then invaginated within 

 the upper surface (Fig. 16). The embryo thus becomes cup- 

 shaped, its walls consisting of two layers : an outer layer, E, 

 formed from the original upper part of the blastula ; and an 

 inner layer, H, consisting of the invaginated cells, which originally 

 formed the lower pole of the blastula. 



Fig. 15. 



Fig. Hi. 



Figs. 15 and 16. — Formation of the gastrula of Amphioxus. The embryos are 

 bisected vertically, one half alone being represented. x 220. (After 

 Hatschek.) 



Fig. 15. — Flattening of the lower pole of the blastula prior to invagination. 

 Fig. 16. — Commencing invagination of the lower pole to form the gastrula. 



B. blastocoel or segmentation cavity. E, epiblast. G, archenteron or gastroccel. H, hypoblast . 



As the invagination proceeds, the blastocoel becomes gradually 

 diminished in size, and is ultimately completely obliterated, the 

 inner and outer layers of the gastrula coming in contact with 

 each other (Fig. 17, H, e). 



The two layers of cells of which the wall of the gastrula con- 

 sists are the two primary germinal layers. The outer layer is 

 spoken of as the epiblast, e, and the cells of which it consists are 

 called epiblast cells : the inner layer is the hypoblast, H, and its 

 cells, which originally were those forming the lower half of the 

 blastula, are called hypoblast cells. 



The cavity of the cup, formed by invagination of the hypo- 



