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GENERAL BIOLOGY 

 w 



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; *', dorsal aorta, /, 

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



