STUDIES IN MAMMALIAN EMBRYOLOGY. 517 



However this may be^ a stage is soon reached — and figs. 11, 28, 

 and 80 are good representations of it — in which the tropho- 

 blast forms a continuous layer over the maternal surface, and 

 over the mouths of the crypts, acting as a sort of pseudo- 

 epithelium. 



This trophoblast layer has at the same time commenced ano- 

 ther transformation, which is of the highest importance for 

 the correct interpretation of the further phenomena of placen- 

 tation. Of this transformation the earliest appearance of the 

 allantoidean trophoblast — even before it is as yet applied 

 against maternal surfaces — has given evidence already. We 

 there notice in the free trophoblast an evident tendency to 

 differentiate into two layers, the inner one of these generally 

 staining more intensely (figs. 75 — 78). This duplicity is also 

 marked in the trophoblastic knobs (figs. 27, 27 a, 78, and 79). 

 We may safely infer that this subdivision of the trophoblast is 

 the same as that which was noticed by van Beneden for the 

 bat, and afterwards by Masius, Duval, and others for the 

 rabbit and other rodents. Van Beneden applied the names of 

 "cytoblast^^ and " plasraodiblast" to these closely contiguous 

 subdivisions of the trophoblast. 



We will follow his example, and henceforth designate in the 

 allantoidean trophoblast of Sorex the inner layer as the cyto- 

 blastic, the outer as the plasmodiblastic one. Both of them 

 rapidly increase in extent and thickness as the trophoblast 

 continues to spread over the maternal surface. Superficial 

 inspection of preparations, as those of figs. 28 and 80, would 

 lead us to the conclusion that the real extent of the tropho- 

 blast was limited to the darkly stained layer and knobs there 

 indicated. Still these only represent the deeper cytoblast. 

 The fact is that the plasmodiblast, which is the outer layer, 

 commences to be fused intimately with the maternal tissue in 

 the stage of figs. 27, 27 a, and 79, and is busily engaged in 

 developing lacunary and intercommunicating spaces just out- 

 side the limits of the darker cytoblast layer (fig. 79). These 

 future blood-spaces are spread out in the plane of the layer, 

 and thus come to be directly contiguous to the actual blood- 



