VERTEBRATE DEVELOPMENT 235 



the Frog, differentiate into dorsal and lateral portions. The latter 

 splits into the outer somatic layer and inner splanchnic layer 

 between which is the coelom. 



All this time the Chick embryo, developing in the area of the 

 blastoderm, is lying flat on the surface of the yolk. There now 

 begins, at about the twentieth hour of incubation, a process of fold- 

 ing, first at the anterior end (head fold), later at the sides (lat- 

 eral folds), and then posteriorly (tail fold), which results in 

 an almost complete separation of the embryo from the yolk sac, 

 so that the latter after a time is attached to the embryo only by 

 the yolk stalk, through which the food material is transported 

 to the young embryo by the embryonic vascular system. 



One of the distinctive features of the development of the Chick 

 as compared with the Frog is that, owing to the position of the 

 blastoderm which is spread out flat upon the surface of the yolk, 

 the rudiments of the various structures such as the coelom, heart, 

 alimentary canal, etc., are formed in right and left halves on either 

 side of the main body of the embryo. These are located in the out- 

 lying or, as they are termed, extra-emrryonic regions of the 

 blastoderm. The process of folding off the embryo from the under- 

 lying yolk, which has been noted above, brings the right and left 

 rudiments of these organs together underneath the embryo, where 

 they unite in the mid-ventral line to form the complete organs. 



By the end of the third day of incubation, the embryo has grown 

 to a considerable size, and many of the organs are well-established. 

 At this stage the entire animal is covered by an emrryonic mem- 

 brane, the amnion, which, starting anterior to the embryo in the 

 ectoderm, has grown back over the animal from the anterior end 

 and also to some extent from the posterior end, with the result that 

 the embryo is enclosed in a definite sac (amniotic cavity) . At this 

 stage of incubation, the embryo does not have a straight, antero- 

 posterior axis, but the head end has a bend (cervical flexure) 

 of almost 90 degrees toward the right. The central nervous sys- 

 tem is well-developed, together with the sense organs such as the 

 eye, nose, and ear. The vascular system is also well-established 

 and functioning. The heart at this time consists of an auricle and 

 ventricle. The auricle receives the blood, containing food material, 

 which has come in from the yolk sac through the large vitelline 

 veins. This blood passes into the ventricle, and then it is forced 

 out through the arteries to all parts of the growing body, and 



