THE PLACENTA. 435 



and partly from the embryo or foetus ; the foetal element being 

 supplied by the wall of the blastodermic vesicle, and by the 

 allantois ; and the maternal element by the part of the wall of 

 the uterus to which the blastodermic vesicle becomes attached. 



In the chick, the allantois (Fig. 101) attains a great size, and 

 forms the respiratory organ of the embryo during the later stages 

 of its development. 



In the rabbit the allantois becomes still larger and more 

 important, subserving nutrition as well as respiration. It be- 

 comes firmly attached to the wall of the uterus (Figs. 148 and 

 170), and then gives off, from its outer surface, vascular tufts or 

 villi into the substance of the uterine wall. The vessels of these 

 villi, which are derived from the allantoic arteries and veins, and 

 are therefore continuous with the blood-vessels of the embryo, 

 lie in close contact with the dilated maternal capillaries of the 

 uterus. The intervening walls between the two sets of blood- 

 vessels, foetal or allantoic, and maternal or uterine, become so 

 greatly reduced in thickness that diffusion readily takes place 

 between the two blood streams, through these very thin parti- 

 tions. In this way the fcetal blood derives nutrient matter 

 from the maternal blood, and gives up to it the gaseous and 

 other excretory matters that are formed in the embryo, as a 

 necessary consequence of the chemical changes associated with 

 its growth and development. 



The actual details of development of the rabbit's placenta 

 are extremely complicated, and the accounts given by different 

 investigators are at variance with one another, even in points of 

 primary importance. The most complete and consistent account 

 is that given by Duval ; it is supported in many important 

 respects by Miuot's investigations, and has afforded the basis on 

 which the following description has been founded. 



The mucous membrane of the uterus, in the unimpregnated 

 condition, is thrown into six longitudinal folds, which project 

 into the uterine cavity, and give it a stellate appearance in 

 transverse section. Of these folds, two (Fig. 168, PK) lie on the 

 side of the uterus next to the mesometrium, or mesenterial fold, 

 MM, which attaches the uterus to the abdominal wall ; these are 

 termed by Minot the placental folds or placental lobes. The 

 second pair, or periplacental folds, P.M. lie at the sides of the 



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