508 A. A. W. HUBRECnT. 



which the yolk-sac is bounded we find that only the area 

 vasculosa remains. 



Now it is undeniable that the area vasculosa increases in 

 surface and its cells in bulk even after it has been delaminated 

 from the surface of the omphaloidean mucosa (figs. 11 — 13). 

 This considerable increase goes on in later stages of pregnancy. 

 Arborescent excrescences (figs. 15, 15 a, 65) contribute to ex- 

 tend its surface yet further, and we would thus be able to 

 argue that all this enlargement was at the same time the ex- 

 pression of a heightened activity, by which the coagulable 

 matter filling the yolk-sac is produced. 



Such a conclusion would, however, to my idea, confuse 

 cause and effect. The unexpected ulterior development of the 

 vascular area on the yolk-sac, and the increase in bulk of the 

 hypoblast cells can hardly be explained as meant to contribute 

 to the production of special contents of the yolk-sac, which 

 would then be available for the growth of embryonic tissues 

 only on condition that the processes were reversed, and that 

 by the same channels that have first deposited the coagulable 

 matter it was next absorbed and conveyed to the embryo. 



The explanation is ever so much more natural that it is the 

 presence of this coagulable matter inside the yolk-sac which 

 has brought about the increased ulterior growth of the vessels 

 of the yolk-sac. 



The upper surface of the latter, in consequence of the pecu- 

 liar growth of embryo and placenta already noticed, has come 

 to be inverted into its own cavity, and over a considerable 

 surface the hypoblast cells beneath the area vasculosa can thus 

 be directly bathed by any fluid that is contained in the yolk- 

 sac. If this fluid has nutritive properties, as in the case of 

 Sorex it may very reasonably be expected to have, then the 

 increased growth of the area vasculosa and the increased eflS- 

 ciency of the apparatus (in casu of trophoblastic annulus and 

 non-placental trophoblast) by which this nutritive matter is 

 prepared and conveyed, is at the same time understood. 



And so the disintegration of the omphaloidean portion of the 

 mucosa may be said to lead indirectly to the production in a 



