2l6 



GENERAL BIOLOGY 



Fig. 137. Diagram of nurture of young through embryonic 

 membranes, a, a young embryo with embryonic membranes 

 beginning as folds or outgrowths of ectoderm and endoderm; 

 b, an older embryo with the folds extending over the back to 

 inclose the embryo; c, an older embryo with the placental 

 attachment to the uterine wall established ;_ d, diagram of the 

 channels of food intake and waste removal in the embryo; w, 

 wall of the uterus; o, o, folds of the ectoderm which when 

 confluent form the outer and inner amnion; t, t, p fold of the 

 endoderm, outgrowing to form the allantois; q, the vestigial 

 yolk sac; r, the amniotic cavi sac, in which the embryo floats; 

 s, gill slits; , u, umbilicus; v, v, fore and hind leg buds; w, 

 the circulation through the placenta; m, indicating the course 

 of the blood of the mother parallel to n, that of the embryo; g, 

 gill circulation of the embryo; h, heart; t, dorsal aorta, ;, 

 post cava; k, allantoic artery; /, allantoic vein 



