FCETAL MEMBRANES OP OPOSSUM AND OTHER MARSUPIALS. 483 



could not be effected if the absorbent villous area were shifting 

 about from one part of the uterus to another. This fixity of 

 position must have been an iaiportant step towards the 

 establishment of an allantoic placenta. 



Although we may now be reasonably certain of the early con- 

 dition of the foetal membranes in the Marsupials, it must be 

 borne in mind that all the latter part of their history is still a 

 blank, and that the allantois in the later stages may enter into 

 very important relations with the subzonal membrane. Bal- 

 four, with his usual discernment, suggested a probable condi- 

 tion among the primitive Placentalia,^ in which the yolk-sac 

 and allantois shared the placental function. I think it is not 

 improbable from the evidence given by Specimen 2, and from 

 the rapid growth of the allantois in the opossum, that this con- 

 dition may yet be found among the Marsupials. The fact that 

 no foetal membranes are brought forth at birth has, I believe, 

 been correctly attributed to the very tortuous vaginal passage 

 through which the young pass in their descent. 



The evolution of the placenta is an interesting subject of 

 speculation, which it is tempting to review, now that we have 

 more light upon the functions of the yolk-sac. 



( 1 .) In the low reptilian for ms which preceded the Mammals there 

 was undoubtedly a substitution of the viviparous for the ovi- 

 parous mode of reproduction, by the gradual reduction of the food 

 yolk and the retention of the embryo in the uterus. The whole 

 character of Mammalian development points unquestionably 

 to the former presence of a mass of food yolk in the ovum, and 

 there is every reason to suppose that the loss of this source of 

 supply was gradually and pari passu compensated by the sub- 

 stitution of the maternal nutrition, so that the embryos were 

 partly nourished by the yolk, partly by a feeble absorption of 



an inch long. All the bodily functions are fully in action, the fore limbs are 

 strong and provided with claws, the young are taken in the mouth of the 

 mother from the valva and placed in the pouch, probably close to one of the 

 nipples, the grasping of which is instinctive. They will retake the nipples after 

 removal from the pouch and exposure for several hours. 

 ' 'Comparative Em, bryology,' vol, ii, p. 216. 



