86 EARLY EMBRYOLOGY OF THE CHICK 



like the yolk, is surrounded by extra-embryonic membranes by 

 which it is absorbed and transferred over the extra-embryonic 

 circulation to the embryo. 



Toward the end of the period of incubation, usually on the 

 igth day, the remains of the yolk-sac are enclosed within the 

 body walls of the embryo. After its inclusion in the embryo 

 both the wall and the remaining contents of the yolk-sac 

 rapidly disappear, their absorption being practically completed 

 in the first six days after hatching. 



The Amnion and the Serosa. The amnion and the serosa 

 are so closely associated in their origin that they must be con- 

 sidered together. Both are derived from the extra-embryonic 

 somatopleure. The amnion encloses the embryo as a saccular 

 investment and the cavity thus formed between the amnion 

 and the embryo becomes filled with a watery fluid. Suspended 

 in this amniotic fluid, the embryo is free to change its shape 

 and position, and external pressure upon it is equalized. Mus- 

 cle fibers develop in the amnion, which by their contraction 

 gently agitate the amniotic fluid. The movement thus im- 

 parted to the embryo apparently aids in keeping it free and 

 preventing adhesions and resultant malformations. 



The first indication of amnion formation appears in chicks 

 of about 30 hours incubation. The head of the embryo sinks 

 into the yolk somewhat, and at the same time the extra-embry- 

 onic somatopleure anterior to the head is thrown into a fold, 

 the head fold of the amnion (Fig. 32, ^4). In dorsal aspect the 

 margin of this fold is crescentic in shape with its concavity 

 directed toward the head of the embryo. The head fold of the 

 amnion must not be confused with the sub-cephalic fold which 

 arises earlier in development and undercuts the head. 



As the embryo increases in length its head grows anteriorly 

 into the amniotic fold. Growth in the somatopleure itself 

 tends to extend the amniotic fold caudad over the head of the 

 embryo (Fig. 32, B). By continuation of these two growth 

 processes the head soon comes to lie in a double walled pocket 

 of extra-embryonic somatopleure which covers the head like a 

 cap (Fig. 29). The free edge of the amniotic pocket retains 

 its original crescentic shape as, in its progress caudad, it covers 

 more and more of the embryo. 



