482 CHORDATA. 
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 A/etatheria 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 Z£utheria 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 Zutheria 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 hemal 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 (of. 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 (dscotdal) 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 
the villi are aggregated into tufts. 
Again, the villi may remain more or less simple processes 
protruding into the maternal tissues, so that at birth they 
can be withdrawn from their pockets, leaving the maternal 
tissues intact, or they may become extremely complex and 
branching and so inextricably interwoven with the maternal 
tissues that parts of the latter have to be shed at birth. The 
former type of placenta is termed on-deciduatze and the latter 
deciduate. ‘The only other alternative is for the embryo to 
leave its share of the placenta (allantois) behind at birth. 
This occurs (Perameles) and the remains of the allantois 
are absorbed by the maternal tissues. This type has been 
termed contra-deciduate. . 
