STUDIES IN MAMMALIAN EMBRYOLOGY. 507 



stands from the first in a particular relation to that ompha- 

 loidean mucosa^ and to the disintegrated products into which 

 it gradually dissolves, and that the cells of the annulus actively 

 absorb the blood corpuscles that are set free during this disin- 

 tegrating process, the hypothesis does not seem a strained one 

 which assumes the annulus to participate in the production of 

 the coagulable matter that is gradually accumulating inside 

 the yolk-sac. 



Only one layer of hypoblast- cells coating the trophoblastic 

 annulus on the inside separates the protoplasm of the latter 

 from the yolk-cavity (figs. 50, 84, and 88). And the slight 

 increase in size of these hypoblast cells where they are applied 

 against the annulus (figs. 50 and 84) rather favours than con- 

 tradicts the supposition that they too aid in transporting 

 matter that is originally outside the yolk-sac towards the inside 

 of it, albeit with changed chemical and physical properties. 



The cells of the annulus merge very gradually, as was already 

 noticed, into those of the non-placental trophoblast. As this 

 is applied against the uterine epithelium, immediately behind 

 which numerous maternal blood-vessels convey the blood from 

 the mesometrium to the placenta (figs. 15 a, 15 b, 56, 57, 

 11 — 13), it is neither a strained hypothesis to suppose that 

 here, too, certain substances transudate from the blood-vessels 

 through the uterine epithelium, are absorbed by the cells of the 

 non-placental trophoblast, and transported through this and 

 through the underlying hypoblast into the cavity of the yolk- 

 sac. Such a regular passage directed inwards through these 

 layers would go far to explain the very regular growth and 

 increase of the coagulable material all round the surface where 

 the non-placental trophoblast is in contact with the uterine 

 epithelium. Trophoblastic annulus and non-placental tropho- 

 blast, continuous anatomically, would then also physiologically 

 play a part to a certain extent analogous. 



We must not forget that if the hypothesis of the derivation 

 of the coagulable matter here enunciated is not accepted, 

 another source must be indicated from whence this matter can 

 have been derived. And if we consider the other tissues by 



