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. OrIGIN 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-embryonic 
blastoderm, and the allantois arises as an outgrowth of the ven- 
tral wall of the hind-gut. 
OL. 
Fic. 72. — The head of the same embryo from 
below. 
a. i. p., Anterior intestinal portal.  B. a., 
Bulbus arteriosus. Inf., Infundibulum. or. pl., 
Oral plate. Tr. a., Truncus 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-amniotie 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 ammniogenous zone immediately adja- 
