LATER DEVELOPMENT OF CHICK AND RABBIT 441 



This is the essential arrangement of the adult placenta, but it is 

 somewhat masked by the facts that in the first place the embryonic 

 capillaries enlarge considerably, and in the second the trophoblastic 

 tissue and allantoic mesenchyme thin out until they become almost 

 negligible. So it is that the embryonic capillaries are practically 

 bathed in maternal blood, and an interchange of substances between 

 the two blood streams is simply a question of diffusion. It is 

 important to notice, however, that in spite of their proximity the 

 two blood streams never actually intermingle, owing to the fact 

 that the embryonal capillaries always retain intact their endothelial 

 walls. 



We see then that the placenta when fully formed is a very intimate 

 union between foetal membranes and uterine mucosa. It has the 

 form of a flattened, disc-shaped (hence the term placenta = a flat 

 cake), spongy, vascular thickening composed of two lobes, one related 

 to each original placental ridge. The embryo is attached to this by 

 means of the allantoic stalk carrying the blood-vessels, and around 

 which the yolk sac stalk becomes slightly twisted to form a common 

 umbilical cord. From its shape this type of placenta is termed 

 discoidal, and it is somewhat similar but not bilobed in man. 

 Owing to the fact that the fusion is so complete, a superficial part of 

 the uterine mucosa, mainly blood, comes away from the deeper 

 layers with the allantoic portion at birth, and so the placenta is 

 spoken of as deciduate. At parturition the amnion and chorion 

 rupture and the embryo is born attached to the placenta and 

 remains of the membranes by the umbilical stalk, which is gnawed 

 through by the parent. The biting squeezes together the blood- 

 vessels and the small remnant soon shrivels up, leaving only an 

 almost indistinguishable umbilical scar or navel, which marks the 

 point where the animal when an embryo was attached to the 

 placenta. 



We have seen then that as the mammal has descended from 

 an ancestor laying a large heavily yolked telolecithal egg, it is provided 

 with a series of membranes on the whole very similar to those of the 

 chick and homologous with them. Their original functions in 

 relation to the absorption of the yolk and protection of the embryo 

 disappear, and they take on entirely different ones, still to a certain 

 extent protective, but in the main concerned with excretion and 

 nutrition. 



