482 CHORD AT A, 



The yolk-sac placenta is a functional organ in the Meta- 

 theria. In them the zonal placental area extends upwards 

 till it covers the greater part of the upper half of the 

 blastocyst and probably largely replaces the prokalymma 

 at the lower pole. The allantois in Metatheria degenerates 

 and eventually loses its connection with the serosa, though 

 in certain forms it may remain attached over a small disc- 

 like area and form, indeed, a true allantoic placenta. 



In the Eutheria this state of affairs is carried still 

 further, and the allantois spreads over a large area of the 

 serosa, throws out villi and forms a large allantoic placenta. 

 The yolk-sac in these forms degenerates ; it eventually 

 loses its connection with the serosa and lies as a small 

 vestige beside the allantoic stalk. Indeed, it is questionable 

 how far in Eutheria the true yolk-sac placenta is formed, 

 for the allantois is developed at a very early stage and tends 

 to become functional as the organ of haemal nutrition, 

 whilst the prokalymma is still functional. In many Eutheria 

 the allantois lines the whole inner surface of the serosa in 

 late stages, just as the yolk-sac tends to do in the case of 

 the Metatheria {cf. Figs. 331 and 342). 



The allantoic placenta attains a far higher standard of 

 perfection than the yolk-sac placenta. In shape we have 

 seen that it originates as a sac or disc {discoidal) from which 

 it may spread over the equator to form the dome-shaped ; the 

 villi may then disappear at the pole and produce the zonary, 

 or the spreading may extend to the other pole and form the 

 diffuse^ a modified form of which is the cotyledonary in which j 

 the villi are aggregated into tufts. 



x\gain, the villi may remain more or less simple processes 

 protruding into the maternal tissues, so that at birth the} 

 can be withdrawn from their pockets, leaving the materna 

 tissues intact, or they may become extremely complex anc 

 branching and so inextricably interwoven with the materna 

 tissues that parts of the latter have to be shed at birth. Th< 

 former type of placenta is termed non-deciduate and the latte 

 deciduate. The only other alternative is for the embryo t« 

 leave its share of the placenta (allantois) behind at birth 

 This occurs {Ferameles) and the remains of the allantoi 

 are absorbed by the maternal tissues. This type has bee: 

 termed contra-deciduate. 



