FROM TWELVE TO THIRTY-SIX SOMITES 135 



vertebrate embryos; the cause appears to lie in the precocious 

 development of the central nervous system, of which more here- 

 after. Only the cranial flexure remains as a permanent con- 

 dition. 



II. Origix of the Embryonic Membranes 



The period from about 12 to 36 somites also includes the early 

 history of the embryonic membranes, amnion, chorion, yolk-sac 

 and allantois. The first three arise from the extra-embrvonic 

 blastoderm, and the allantois arises as an outgrowth of the ven- 

 tral wall of the hind-gut. 



-Pr'am. 

 V.Ao. 



Fig. 72. — The head of the same embryo from 



below. 



a. i. p., Anterior intestinal portal. B. a., 

 Bulbus arteriosus. Inf., Infimdibulum. or. pi., 

 Oral plate. Tr. a., Trunciis arteriosus. 

 Ven., Ventricle, v. Ao., Ventral aorta. 



Origin of the Amnion and Chorion. The amnion is a thin 

 membranous sac, forming a complete investment for the embryo 

 and continuous with the body-wall at the umbilicus; it lies beneath 

 the chorion to which it remains attached throughout incubation 

 by a plate of tissue (sero-amniotic connection), and it arises in 

 common with the chorion from the extra-embryonic somatopleure. 

 The entire somatopleure external to the embryo is used up in 

 the formation of these two membranes. The amnion arises from 

 a portion immediately adjoining the embryo itself; the remainder 

 of the somatopleure peripheral to the amniogenous part forms 

 the chorion. Thus the extra-embryonic somatopleure may be 

 divided into two zones; an amniogenous zone immediately adja- 



