DEVELOPMENT OF LISSEXCEPHALA. 720 



third kind of placenta more exclusively related to the nutrition of 

 the foetus. For this grows so quickly and becomes so large, 1 that 

 the allantoic placenta, serving both for respiration and nutrition, 

 needs the help it obtains through absorption, by the vitelline 

 fringes, of the uterine nutrient matter in which thev are bathed. 



O ' * 



The umbilical cord, as in most Rodents, is very short and thick. 



In the Aguti the decidual placenta is still more reduced, little 

 more than the originally traversing maternal vessels remaining 

 on their passage to the later allantoic placenta, which they seem 

 to suspend by a central part, opposite to which, on the other side 

 of the placenta, the foetal vessels proceed. In the Rat the ma- 

 ternal or decidual placenta is cotyloid, and is adapted to a small 

 convex process of the centre of the uterine surface of the button- 

 shaped foetal placenta; 2 and, as in the Guinea-pig, the allantois, 

 after laying the foundation of the latter, speedily disappears. 

 The like happens in the Water-vole, in which the foetal placenta 

 is small and circular, convex toward the uterus and flat toward 

 the vitelline chorion, which has its attachment limited to the 

 central part of the placenta! disc. 



In the Mole and Shrew the vitellicle is large, and supplies the 

 o-uter envelope of the ovum with vessels, coalesces with, and seems, 

 indeed, to form it. The allantois, bending to the dorsal aspect of the 

 embryo, carries its vessels to that part of the amorphous mass of 

 decidua enveloping the ovum. The early embryo in its amnios 

 thus appears to be suspended by opposite poles formed respectively 

 by the vitelline and allantoic trunks. The allantoic vessels or- 

 ganise the fine villi of the foetal placenta in a small proportion of 

 the thick deciduous mass : this in the growth of the embryo be- 

 comes reduced to a subcircular maternal disc, larger than the foetal 

 one, which is imbedded in its central concave surface. The area 

 on the peripheral convex surface affording the maternal supply of 

 blood, is small in proportion to the placental disc. The terms 4 foetal ' 

 and ' maternal' relate to the source of the main part of the vascular 

 supply of such divisions of the discoid placenta. The maternal 

 vessels, the orifices of the veins being conspicuous on the area of 

 placental detachment, are continued into the allantoic or foetal 

 button, and the villi of this part extend into the decidual part of 

 the maternal placenta. Beyond this the decidual substance 

 becomes reduced to a very thin layer, traceable over part of the 

 chorion. The convexity of the maternal placenta is continued 

 with the lining of the uterus. Such lining is homologous in tissue 



1 Calling in the Guinea-pig for the special expansion of the pelvis shown in Vol. II. 

 p. 380, fig. 246. 2 xx. vol. v. p. 117, no. 3467. 



