SIMPLIFIED DIAGRAM (LONGITUDI^'AL SECTION) TO ILLUS- 

 TRATE THE MEMBRANES OF THE CHICK EMBRYO 



The dotted lines represent mesoblast. The diagram shows the state 

 of development after about ten days. The embryo itself is in the 

 central part of the diagram. Immediately above the gut is the noto- 

 chord (shown in black), and immediately above that is the nerve-cord, 

 of which the right end is the brain. The two " umbilical cords " 

 mentioned by Aristotle (III. 753 b 20 £f.) are shown : (a) the yolk-sac 

 stalk, (b) the stalk of the allantois. 



To begin with, the embryo is a sort of thin plate on top of the yolk ; 

 and as time goes on, both the amniotic cavity (which encloses the 

 embryo) and the allantois (which acts as a respiratory organ and as a 

 receptacle for excreta) progressively encircle the yolk, which finally 

 becomes enclosed in the embryo (as Aristotle says). The chorion and 

 allantois coalesce after a period, and the resulting chorio-allantois then 

 corresponds to the fetal placenta of mammals. The chorion is really 

 the outer layer of the amnion. The extra-embryonic coelom, wliich 

 is lined with mesobla.st, is an extension of the coelom proper (the 

 main body-cavity), which is also lined with mesoblast. 



369 



