THE FOETUS AND UTERUS. 97 



developmental changes which occur in the body-stalk are of the 

 greatest practical importance (p. 99). 



The Chorion and Allantois. — At this early stage a diverti- 

 culum has arisen from the hind gut below the caudal part of the 

 embryo (Fig. 75). This ventral evagination of the hind gut, 

 which grows along the body-stalk, carrying the splanchnopleure 

 with it, forms the allantois. The allantoic bud, which is hollow 

 only at its basal part, spreads out on the inner surface of the 

 prechorion, forming a lining to it. The prechorion or false 

 chorion, with the addition of the allantois, forms the chorion, or 

 true chorion. The posterior ends of the two primitive dorsal 

 aortae, which terminate until now on the yolk sac, are carried out 

 with the allantoic bud and distributed within the villi of the 

 chorion. The posterior ends of the two dorsal aortae thus become 

 the hypo-gastric and umbilical arteries (Young and Eobinson). 

 The veins which return the blood become the umbilical veins. 

 At first there are two of them, and they return the allantoic blood 

 direct to the sinus venosus, afterwards to the ducts of Cuvier, 

 and thus to the heart (Fig. 190, p. 232). In this way the 

 foetal circulation is set up. The heart pumps the blood into the 

 chorionic villi ; these are imbedded in the maternal decidual 

 covering; the blood is returned by the umbilical veins (Fig. 76). 

 At this stage, and indeed until the end of the 6th week, the 

 membranes, with the embryo within them, can easily be detached 

 from the decidual nest in the uterus, and then appear as a villous 

 vesicle, the chorionic vesicle, about the size of a pigeon's egg. 

 The allantois never forms a free vesicle in man nor in the higher 

 primates. It occurs as a vesicle in other mammals, birds and 

 reptiles, in which it has a double function : 1st, to form a 

 respiratory medium, as is also the case in man ; 2nd, to form a 

 reeeptaculum for the secretion of the kidney. 



Formation of the Placenta. — The condition of the mem- 

 branes in the 3rd month (Fig. 76) differs from that of the 1st 

 month (Fig. 75) by the formation of the placenta. In the first 

 month the chorion is uniformly covered by shaggy villi, which 

 project into the decidua and draw sustenance for the embryo 

 therefrom. This is the permanent condition in low primates 

 (Lemurs). In man the chorionic villi which project within the 

 decidua serotina hypertrophy, while those within the decidua 



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