FCETAL NUTRITION: THE PLACENTA 377 



blast at the time of its first attachment. Hubrecht, on the 

 basis of Huxley's statement that Insectivora are among the 

 most archaic of Mammals, has investigated several members of 

 this order as showing probably the most ancient type of placenta, 

 and thus affording a starting-point for a classification. Accord- 

 ing to Huxley, the least differentiated types, the hedgehogs 

 and Gymnura, occupy a central position, while shrews show 

 resemblances to Rodents, and Tupaiw to lemurs ; moles and 

 Gahofitheci vary in other directions, while the whole order 

 shows more general relationships to Carnivores and Ungulates. 

 But at present these relationships are not understood. It 

 seems impossible to trace any connection between the placenta 

 of the sheep, in which there is no circulation of maternal blood 

 in the foetal parts of the placenta but the foetus is nourished 

 by uterine milk, and that of the hedgehog, in which maternal 

 blood circulates in the trophoblastic lacunae and forms the 

 main source of nutriment. 



At present, we must be content with a review of the pro- 

 cesses 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.^ 



PART III 



THE FCETAL MEMBRANES, THE YOLK-SAC, 

 AND THE PLACENTA 



I. General Anatomy op the Foetal 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 



' Throughout this chapter, the arrangement of the mammalian orders is 

 more in accordance with the older views of placental classification, but an 

 attempt has been made to emphasise the trophoblastic characteristics. 

 Since it was written, an important memoir has been published by Hubrecht 

 ("Early Ontogenetic Phenomena in Mammals," Quar. Jour. Micr. Sci„ 

 1908), in which he follows out, in more detail than previously, his ideas 

 regarding the phylogeny of the placenta. 



