4IO THE PHYSIOLOGY OF REPRODUCTION 



more general relationships to Carnivores and Ungulates. But at 

 present these relationships are not understood. It seems inipossible 

 to trace any connection between the placenta of the sheep, in which 

 there is no circulation of maternal blood in the fcetal 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 soiirce of nutriment. 



Hubrecht' has contributed an important memoir setting out in 

 some detail his views on the phylogeny of the placenta and its 

 classificatory value, and Assheton ^ has expressed himself as in 

 agreement with the general trend of Hubrecht's ideas. According 

 to these authorities we have on the one hand in the Ungulata, 

 Sirenia, Cetacea, and some Edentata a great lateral . expai;ision of 

 the trophoblast, leading to the . " plicate " type of placenta, and on 

 the other hand in Insectivora, Cheiroptera, and Primates cell 

 proliferation of the trophoblast producing thickenings thereof and 

 leading to the "cumulate" type of placenta. The differences between 

 the two types are summarised by Assheton as follows : — '■ 



Cumulate Placenta. Plicate Placenta. 



Great radial proliferation of tropho- Great tang9iitial proliferation . of 



blast, leading to thickening of the ti^ophoblast, leading to folding of 



membrane. the membrane. 



Greater destruction of maternal tissue. Lesser destruction of maternal tissue. 



Much bleeding of mother. , Little bleeding of Inpther. 



Lacunisation of trophoblast. No lacunisation of trophoblast. 



Secretion of uterine glands of less use, Sefetetion of uterine glands of prime 



or in some cases of no use, to the importance.. 



foetus. 



Degeneration of uterine epithelium Little or no degeneration of uterine 



and glands severe. epithelium and glands. 



Embryo or embryos seldom fill the Embryo or embryos usually fill the 



whole cavity of the uterus., whole cavity of the uterus. 



According to Assheton the reason for the causation of the latter 

 character is , that " fixation is brought about or other intimate 

 connection in the cumulate type, while fixation in the plicate type 

 depends more upon the infernal hydrostatic pressure of the blastocyst 

 upagainst the uterus^much as a pneumatic tyre retains its position 

 in the iron rim of a bicycle wheel." 



The Carnivora are regarded as a central group wjth respect to 

 placentation from which either cumulate or plicate type could be 

 evolved. In this connection it may be recalled that in the oestrous 

 cycl^ of the dog there is a normal pseudo-pregnant period in the 

 absence of gestation, and that this is to be regarded as a primitive 

 characteristic. 



1 Hubrecht, " Early Ontogenetic Phenomena in Mammals," Quar. Jour. Micr. 

 Science, vol. liii., 1908. 



2 Assheton, " Professor Hubrecht's Paper, etc.," Quar. Jgw. Micr. Science, 

 vol. liv., 1909. 



