FCETAL NUTRITION: THE PLACENTA 483 



traversed by the materiak on their way to the new organism is 

 the placenta, a complex organ composed of speciaUsed maternal 

 elements and newly developed foetal elements. Among the 

 Monodelphia no uniform plan is observable in the formation of 

 the placenta, nor is it possible to trace each step in its evolution. 

 But Duval's conception of this temporary organ as a maternal 

 hsemorrhage surrounded by foetal elements, and Hubrecht's 

 discovery of such a type of placenta in a mammalian order 

 which is among the most archaic, lead to a change in the ideas 

 of placental classification. We can no longer depend on the 

 shape of the placenta, or the characteristics of the after-birth, 

 for an understanding of its morphological or physiological 

 features. Rather must we go back to the phenomena to be 

 observed in the uterine fixation of the blastocyst, and even 

 earher in the preparation for that fixation. At, this stage we 

 find two constant features, one maternal and the other foetal. 

 The maternal change consists of an epithehal, connective tissue 

 or endothehal proliferation, the trophospongia, which is 

 " specially intended for the fixation of the blastocyst." Ac- 

 cording to Hubrecht, it degenerates into a symplasma when the 

 fixation is accomphshed and the foetal elements are in contact 

 with circulating maternal blood. But its degeneration is not 

 completed at that stage. Though individual cells may die, 

 other cells are formed and take their place, at least in Man, 

 throughout the greater part of pregnancy. Moreover, the cells 

 have other functions to perform. Whether or not they act as 

 a defence against the excessive penetration of the trophoblast, 

 they continue in the rabbit to exercise the glycogenic function 

 for the developing organism till the hepatic cells have attained 

 the power, and there is reason to believe that they play a part 

 in the iron metabolism of the foetus. 



The embryonic preparation is the proliferation of the whole 

 or part of the extra-embryonic ectoderm, the trophoblast, in 

 the spaces of which maternal blood circulates. The outer layer 

 is plasmodial, and thus resembles the maternal symplasma in 

 histological appearance, but differs from it in being a live tissue 

 while the other is dying or dead. The fusion of the trophoblast 

 and trophospongia constitutes the placenta, which is perfected 

 by an increase in the number and size of the trophoblastic 



