86 EARLY EMBRYOLOGY OF THE CHICK 



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

 -''/ which it is absorbed and transferred over the extra-embryonic 

 I circulation to the embryo. 



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

 19th 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 

 u ^'investment and the cavity thus formed between the amnion 

 i%nd the embryo becomes filled with a watery fluid. Suspended 

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

 / and position, and external pressure upon it is equahzed, Mus- 

 I 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 

 'I preventing adhesions and resultant malformations. 

 ^ The first indication of amnion formation appears in chicks 

 ^-^oi 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, ^). In dorsal aspec'f 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-cephaKc 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 

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

 embryo (Fig. 32, ^). 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. 



