FCETAL NUTRITION: THE PLACENTA 509 



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 plasmodi&l, 

 and thus resembles the maternal symplasma in histological appear- 

 ance, 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 lacume, and in the amount of 

 maternal 1 >lood in contact with it. 



The above description does not, however, fit the placenta of 

 Ungulates, for in them the trophoblast is not permeated by maternal 

 blood. If the Insectivore placenta represents the primitive type, or 

 is nearer to it than any qther at present existing, we must assume 

 that the Ungulate placenta, differing more widely from the original 

 type, has lost this characteristic. Further, the placenta of the pig 

 must have undergone a greater degree of modification than that of 

 the sheep. In other words, the old ideas of placental evolution, 

 based on the researches of Turner and others, must l>e literally 

 reversed. The Primates must stand with the Insectivores near the 

 primitive type, while the sheep and pig are near the opposite end, 

 where some of the Didelphia are placed. Such considerations as these 

 must inevitably come up for discussion in all future investigations. 



B. The Nature of the Troplwhlnxtii- J //>//// 



During the period of gestation, the mother organism is concerned 

 with the provision of material for the growth and development of the 

 fertilised ovum and the new-born young. Does the material 

 provided for the ovum, and secured for it by the trophoblast, come 

 from the maternal tissues or from the food supply ? There is no 

 doubt that in insufficient nutrition the tVrtus draws on the tissues of 

 the mother (Jiigerroos 1 ), and a study of comparative placentation 

 goes far to prove that this is a normal process in some orders. It is 

 obvious that such occurs in the earliest stages. In all orders, before 

 fixation of the hlastocyst to the uterine mucosa, the degenerating 

 ovarian cells which surround the extruded ovum form a store of 

 nutriment. In some animals, however, such as the opossum, in 

 which no attachment of the blastocyst can be said to occur, and the 

 sheep, in which the attachment is long delayed, this nutriment is 



1 Jagerroos, "Der Eiweiss-, Phosphor-, uncl Salzumsitz wahrend der 

 Graviditat," .1 ///. f. '//////., vol. Ixvii. 



