GENERATION AND DEVELOPMENT 



707 



possesses within it two tubes — one dorsal (the neural canal) , the 

 other ventral (the alimentary canal). The latter opens in front 

 and communicates with the yolk sac (Fig. 247), and between 

 the two is the mesoblast. 



Nutrition of the Embryo. — The development of the embryo 

 need not be described beyond this point ; what has been intro- 

 duced of the subject is to illustrate the important features of 

 the source of the various body tissues, and the method by which 



Fig. 247. — Diagram of a Longitudinal Section of a Mammalian 

 Ovum, after the Completion of the Amnion (Schafer). 



the early embryo is nourished. Up to the seventh week in the 

 embryo of the mare the nourishment is entirely carried out by 

 the yolk sac. This structure may persist after completing its 

 function, as in the case of the dog. The nourishment contained 

 in the yolk sac of the mammal is not true yolk, but uterine 

 milk derived from crypts in the wall of the uterus. 



When the yolk sac is exhausted, the nutrition of the embryo 

 is otherwise provided for — e.g., it is brought into communica- 

 tion with the blood of the mother, and the blood circulating 



