232 OUTLINES OF CHORDATE DEVELOPMENT 



within which the embryo may fully extend. In general the 

 development of the embryo occurs progressively from the 

 anterior end, posteriorly. Thus the brain and other structures 

 of the head are very large and in a fairly advanced stage of 

 development, while the posterior part of the trunk and the tail 

 are still in process of formation. The heart, which appears very 

 far forward, is also a very prominent feature of the early embryo. 

 From about the fifth day the development and enlargement of 

 the body region reduce the relative prominence of the head 

 region, and the typically bird-like form of the embryo is ac- 

 quired about the eighth day. 



On the twentieth day, usually, the chick makes a small perfora- 

 tion through the shell and begins to breathe with its lungs, and 

 on the following day the young chick breaks entirely from the 

 shell. The chick is representative of those birds whose young 

 are " precocious," for it is able to run about actively, to pick up 

 food, and to lead a generally active life, within a few hours after 

 hatching. 



I. THE EGG AND ITS PRODUCTION 



1. The Egg 



We shall find it convenient to describe first the general struc- 

 ture of the hen's egg in its newly laid condition, although this 

 is not the true ovum; for the "egg" is not laid for some time, 

 usually twenty-one to twenty-three hours, after fertilization, 

 and during these hours the process of cleavage is completed, 

 gastrulation and germ layer formation are well advanced, so that 

 the "egg" already contains a multicellular embryo. 



The true ovum, or egg cell proper, is the large yolk, or 

 vitellus, surrounded by a thin but rather tough vitelline membrane 

 (Fig. 85). The extreme animal pole is nearly free from yolk and 

 appears at the time of laying as a circular whitish area, the 

 blastoderm, 3-4 mm. in diameter: this pole is the less dense and 

 is therefore turned upward when the vitellus is free to rotate. 

 The blastoderm of this stage will be fully described later, but we 

 may now distinguish in it two regions, a central translucent 



