44 



CHORDATE ANATOMY 



At the close of the gastrula period (Fig. 38D) the embryo is an 

 elongated ovoid, the slightly larger end being anterior while the now 

 very narrow blastopore marks the posterior end of the long axis. So 

 rapid is development that this stage is attained about seven hours after 

 fertilization. 



Significance of the Gastrula. The gastrula is the animal in its bare 

 essentials. The outer layer, ectoderm, is potentially protective and 



ANIMAL POLE 



I 



VEGETAL POLE 



EC\ 



Fig. 38. — Gastrulation in AMPHIOXUS. The figures represent sections through 

 the polar axis of the embryo. A, blastula with vegetal region flattened; B and C, earlier 

 and later stages of invagination of vegetal hemisphere; D, gastrulation completed; with 

 elongation of the gastrula, its long axis becomes the horizontal antero-posterior axis 

 of the embryo. A, archenteron; B, blastocoele; BP, blastopore- EC, ectoderm; EN, 

 endoderm; P, polar body. (After Cerfontaine.) 



nervous. It gives rise to the essential outer part of the adult skin, which 

 produces so many important protective structures, and to the whole 

 nervous system, both peripheral and central. The inner layer, endoderm, 

 is nutritive. The cavity within it is the primary digestive cavity or 

 archenteron. It is significant that the wall of the archenteron is derived 

 from the vegetal hemisphere of the blastula. Thus, appropriately, the 

 greater quantity of yolk comes to lie in the lining of the embryonic diges- 

 tive cavity. In the vertebrates the blastopore never becomes mouth and 



