FCETAL NUTRITION: THE PLACENTA 411 



Hubrecht considers the extreme cumulate placenta to be more 

 primitive than the extreme plicate one, but his view is based on 

 the assumption of a non-Sauropsidan origin of Mammals with which 

 Assheton is unable to agree. The Lemurs are described by Assheton 

 as " pseudo-plicate," having been derived separately from Mammals 

 with a cumulate placenta. The Marsupials are regarded as being 

 specialised descendants of placental Mammals (Hubrecht). 1 



In this chapter we must be content with a review of the processes 

 occurring in several Mammals which have been more particularly 

 investigated, without straining to find how such processes have 

 arisen in the course of placental evolution. The arrangement of 

 the mammalian orders is more in accordance with the older views 

 of placental classification, but nevertheless an attempt has been made 

 to emphasise the trophoblastic characteristics. 



PART III 



THE FGETAL MEMBRANES, THE YOLK-SAC, 

 AND THE PLACENTA 



I. GENERAL ANATOMY OF THE FCETAL MEMBRANES 



So far no reference has been made to the part played by the 

 mesoblast in the nutrition of the embryo. The placenta has been 

 described as an organ consisting of maternal and foetal elements 

 of modified uterine mucosa, and trophoblast which absorbs nutritive 

 material from the mucosa and from the maternal blood. The 

 nutriment serves in part for the nutrition of the trophoblast itself, 

 and in part for the growth and development of the embryo. In 

 the earliest stages there are as yet no embryonic vessels, and the 

 nutriment is transmitted from cell to cell. But as the embryo 

 increases in size and its requirements grow in proportion, such a 

 path becomes inadequate, and a vascular channel is developed in 

 connection with the two foetal membranes the yolk-sac or umbilical 

 vesicle, and the allantois. 



The mammalian yolk-sac has only a secondary importance for the 

 nutrition of the embryo. The blastodermic vesicle at an early stage 

 of development is divided into an embryonic and a non-embryonic 

 area. The latter is the yolk-sac which gradually becomes folded off 

 from the embryo. Its relations are the same as those of the yolk-sac 

 in Sauropsida, but the contents are an albuminous fluid instead of 

 yolk. It is commonly believed that the placental Mammals are 



1 See also Hubrecht, ' ; Is the Trophoblast of Hypoblastic Origin, as Assheton 

 will have it ? " and " The Foetal Membranes of the Vertebrates," Quar. Jour. 

 Micr. Science, vol. lv., 1910. 



