CHAP, xm PLACENTA 613 



sac " (umbilical vesicle) cut away (left of the figure) to show the underlying 

 amnion. In B, part of the uterine wall has been removed and part of the yolk- 

 sac and allantois cut out so as to expose the foetus and placenta. 

 a. amnion ; all.-r. dotted line indicating margin of allantois (compare Figs. 166 

 and 168, to) ; all.-u. line along which the allantois is reflected over the placental 

 region ; ch. chorion ; ds. yolk-sac ; dsn point at which the umbilical vessels 

 branch out from the umbilical stalk ; mes. mesentery of uterus ; muc. mucous 

 membrane pt uterus ; pi. foetal surface of placenta covered by the distal wall of 

 the allantois ; rs. circular marginal vessel of area vasculosa of yolk-sac ; u-h. 

 undilated narrow portion of uterus between two foetus-containing swellings ; 

 u-l. lumen of uterus ; v. om. vitelline vessels ; v. u. allantoic vessels. (From 

 Grosser's VerL Anal. *. Entwick. geschichte der EihauU und der Placenta.) 



blood by the allantoic vessels, and the blood-supply of 

 the uterus is at the same time greatly increased : the 

 foetal and dilated maternal capillaries and sinuses are 

 thus brought .into intimate relation with one another in 

 the placenta, being only separated by thin layers of 

 epithelium : diffusion can thus take place between 

 them, nutrient matter and oxygen diffusing from the 

 blood of the mother into that of the foetus, while excre- 

 tory substances pass from the blood of the foetus into 

 that of the mother. 



The disc-shaped or discoidal placenta of the Rabbit 

 is of the type termed deciduate, its villi being so 

 intimately connected with the uterine mucous mem- 

 brane that a part of the latter comes away with it at 

 birth in the decidua, or afterbirth, which is attached to 

 the newly-born young by the umbilical cord, consisting 

 of the stalks of the allantois and flattened yolk-sac 

 twisted together (Figs. 167 and 168). 



The blood-vessels at the plane of separation of the 

 placenta and uterus are very small, and the uterus 

 contracts so rapidly when the young are expelled at 

 parturition that very little bleeding takes place, and 

 the mucous membrane rapidly undergoes regenera- 

 tion, ready for the next conception. The cord is 

 gnawed through by the parent rabbit, the blood-vessels 

 being compressed in the process ; and it soon shrivels 

 up and comes away at the navel or umbilicus (see p. 597) 



