26 



Growth in length of the Vertebrate Embryo 



In Anamnia the archenteron is formed at the same time that 

 the deuterogenetic centre begins to come into existence. 



In Amniota the archenteron is formed before the deuterogenetic 

 centre has come into existence. 



In Anamnia the archenteron is formed either by invagination 

 of one wall of the blastula within the other, or by a process of 

 splitting or delamination, producing a split which is always open 

 to the exterior, or by a combination of both processes. In 



a 



Fig. 13. Diagram to show the opening of the blastopore in Anamnia the line 

 a b divides protogenetic tissue (left) from deuterogenetic (right). 



Dots and fine lines protogenetic endoderm and ectoderm respectively; broken 

 lines = deuterogenetic endoderm; thick lines = deuterogenetic ectoderm; the 

 bracket on the right marks the metenteric opening, or notopore (Brachet's 

 "blastopore reel"), the bracket to the left marks the archenteric opening 

 ('* blastopore virtuel "). 



Amniota the archenteron is formed by internal secretion of a 



, 



cavity among or within the yolk cells. 



These differences are in correlation with a physiological process 

 which differentiates the mode of development very sharply 

 indeed from that of the Anamnia. This is the fact that in an 

 early stage of development, a fluid is secreted into the two great 

 cavities of the embryo, namely the archenteron and the coelom. 



This condition of oedema may be said to be the physiological 

 essential cause of the formation of the amnion and allantois, 

 which are the most distinguishing features of the Amniota as 

 compared with other vertebrates. If this is so we can see why 



