STUDIES IN MAMMALIAN EMBRYOLOGY. 515 



first formed as the placeuta yet further increases in size and 

 in thickness. 



Simultaneously with this growth of secondary and tertiary 

 allantoideau villi, the thick layer of trophoblast in which their 

 insertion took place furnishes the material for the blood- 

 cavities by which these extending villi are being surrounded. 

 That same trophoblast layer has in the further stages (figs. 14 

 and 15) no longer such a considerable thickness, and we can 

 now understand how there can be no doubt that all the tissues 

 that contribute to the formation of the part of the placenta 

 which is marked in black are of embryonic origin. 



During the considerable increase in superficial extension of 

 the placenta the cryptal region has at the outset not remained 

 quite stationary, though its increase is in no comparison what- 

 ever to that of the trophoblastic region. Its growth is more 

 adapted to the obvious fact that it has to spread over an ever 

 enlarging area. This leads to a decrease in thickness that is 

 very obvious when we compare the figs. 13, 14, and 15 with 

 each other. And whereas in the first-named figure the crypts 

 are yet very clearly observable, this is much less the case in 

 figs. 14 and 15, the whole of the cryptal and intercryptal 

 tissues being here only represented by a layer of deeply stain- 

 ing nuclear matter and nuclear detritus. 



After having given this summary description of the important 

 modifications that go on in the placentary region, we shall have 

 to analyse in further details the processes that cause them, and 

 to furnish the confirmation of the interpretation here given. 

 We must then go back to the stage of uterus Nos. 42 and 45 

 (figs. 24, 25, and 74). 



We here have the mucosa before us of the placental region 

 in the phase of its highest development, anteriorly to the 

 adhesion of the allantoidean trophoblast against it. The epi- 

 thelial crypts, that have originated by the proliferation above 

 described (p. 493), are in possession of a very distinct epithelial 

 lining, passing continuously into that of the uterus lumen. They 

 are closely pressed together,and some of them bifurcate towards 

 the outer circumference, the blind ends of the crypts being 



