522 A. A. W. HTJBREOHT. 



that such blood-spaces not only arise in those nests of nuclei, 

 but also between them. The whole of the set of blood-spaces 

 in the syncytial tissue here referred to is thus gradually 

 developed by processes that, though closely comparable, are 

 not yet identical in the earlier and in the later phases of 

 pregnancy. The final point to which they lead up, and which 

 we find represented in the fully ripe placenta (figs. 32 and 

 52 — 54), is this, that the lower surface of the placenta where 

 the vessels of the allantois are applied against it very much 

 resembles the phase of fig. 90, but that the rest of the tropho- 

 blastic tissue is so exceedingly attenuated between the allan- 

 toidean villi and their ramifications that by the stretching of 

 the syncytium only a very thin partition of trophoblast, with 

 nuclei regularly distributed in it (figs. 52, 53, and 86), sepa- 

 rates the maternal blood from the embryonic. 



The latter, circulating in the allantoidean villi, is surrounded 

 by the tissue of the villus, which in the earlier stages is indeed 

 substantial (figs. 85 and 86). There is to each villus a central 

 blood-space (figs. 85 and 30) and numerous peripheral ramifi- 

 cations immediately under the surface (figs. 81, 82, 85, and 86). 

 But as the placenta increases in size, the trophoblast in tenuity, 

 and the villi with their ramifications in length, this primarily 

 more massive tissue is also extraordinarily stretched, and 

 finally not more thau the thickness of one cell ensheaths the 

 embryonic corpuscles in the villi. 



I have no doubt that in the fully ripe placenta even this 

 covering disappears as far as the finer ramifications are con- 

 cerned, and that there only the thin trophoblastic partition 

 above referred to separates the maternal from the embryonic 

 blood. 



We may conclude from the foregoing that the passage of 

 blood from the maternal vessels into the embryonic trophoblast 

 takes place in the shrew at a later period of the development 

 than in the hedgehog. For this passage communications must 

 necessarily originate (p. 35) between the maternal blood-vessels, 

 and the embryonic lacunary spaces which are intended for the 



