434 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



upper ends of the oviducts. Development begins as the fertilized 

 ovum passes down the oviduct to the uterus, where it becomes 

 attached by the placenta, through which it receives its nutrition 

 from the blood of the mother. Hence, although the development 

 of the mammalian embryo itself resembles that of a bird or reptile, 

 the method of nutrition is wholly different. The blood vessels of 

 the embryo extend out to a system of capillaries in the part of the 

 placenta originating from the embryo, where they parallel capil- 



Fig. 228. — Development of opossum. Left, embryo a few hours before birth, 

 showing rudimentary development of head and other parts of body as com- 

 pared with fore limbs which are used for climbing to marsupium or brood 

 pouch of mother. Right, an opossum and a few of her young at a stage after 

 they have left the marsupial pouch. 



(Embryo after Hensen, Journal of Anatomy, Vol. 28; Mother and young, photo, by 

 courtesy American Museum of Natural History.) 



laries within the maternal portion of the placenta (Fig. 237, p. 

 453) . There is no actual confluence of the blood of the foetus, as 

 such an embryo is called, and the blood of the mother, although it 

 is often supposed that this is the case. Nutrient materials merely 

 diffuse from the mother's blood into that of the foetus, and waste 

 products pass in the opposite direction. The embryo appears to 

 be as independent of the mother as though it were a parasite Uving 

 at the expense of her body. Its circulatory system is its own, and 

 all the other parts are clearly those of a separate organism. 



As a matter of Comparative Embryology, the exact corre- 

 spondence between the amnion that surrounds the foetal mam- 

 mal and that surrounding the embryonic bird or reptile may be 

 noted (c/. Figs. 227 and 234). The chorion and the allantois 



