FCETAL NUTRITION: THE PLACENTA 373 



the four-cell stage of Macacus nemestrinus, it had already dis- 

 appeared. 



With regard to its functions, there is Httle doubt that the 

 degenerating cells of the corona radiata, and later the albumen 

 layer, serve as food for the growing mass of the ovum in the 

 Fallopian tube and uterus. In the investment in the mouse, 

 Jenkinson ^ found nutritive substances — fat, and probably also 

 protein matter. In addition, Bonnet has adduced strong 

 evidence to show that it is absorbed by the ectoderm of the 

 blastodermic vesicle. In the rabbit the albumen layer forms 

 a tough, strong membrane enclosing at the end of the third day 

 the sohd morula. Within the mass of cells a cavity develops 

 and rapidly increases by diffusion inwards of fluid. " It is 

 hardly conceivable that the dehcate cells could cause expansion 

 of the tough albuminous wall. Rather the osmotic current is 

 more inwards than outwards, either simple or more probably 

 assisted by the vital activity of the cells " (Assheton). Heape 

 had previously pointed out that the increasing fluid must be 

 secreted into the interior of the blastocyst under considerable 

 pressure, as the vesicle remains spherical and extends the uterine 

 walls before it. Once inside, the fluid exerts a greater or less 

 hydrostatic pressure, which is counteracted by the albumen 

 layer, and the rupture of the vesicle is prevented. At the 

 beginning of the cavity formation in the morula, the cells are 

 not yet pressed oh by the investment. Later the vesicle in- 

 creases in size, and the outer cells are pressed and flattened. 

 At the same time the albumen layer is thinned, and is soon 

 hardly perceptible. Finally it ruptures, and immediately after- 

 wards the blastodermic vesicle is flaccid, apparently from injury 

 to its wall. 



Besides its nutritive and protective function, the investing 

 layer may prevent the contact of the external cells of the blasto- 

 dermic vesicle with the cells of the uterus. Only when it has 

 disappeared is fusion of the maternal and foetal elements 

 possible. Robinson has followed out this idea in different 

 Mammals. He suggests that in those animals (Carnivores, 

 rabbit) in which the embryonic ectoderm reaches the surface, 



' Jenkinson, " Observations on the Physiology and Histology of the 

 Placenta of the Mouse," Tijd. Nederl. Dierk., Ver. ii., Dl. 7. 



