416 JAS. P. HILL. 



— The vascular omphalopleure is on the whole similar to that 

 of Stage C. Here and there, however, the flattened cells of 

 the entoderm give place to larger somewhat cubical cells with 

 rounded free ends. In the protoplasm of some of these larger 

 cells there occur vacuoles. Over the sinus terminalis and the 

 larger vessels of the vascular area the entoderm cells are also 

 markedly enlarged, and much more so than in Stage C. The 

 entoderm cells in these positions are now somewhat club- 

 shaped in form, with their enlarged ends projecting freely and 

 containing the ovalish or rounded nuclei (fig. 24, ent. over 

 s. t.). Selenka has already described a similar condition of the 

 entoderm cells of this region in Didelphys. He says (2, 

 p. 138), '■' Die Entodermzellen des Chorion verandern gleich- 

 falls vielfach ihre Gestalt wahrend der letzten zwei Tage des 

 Foetallebens. Sie werden cylindrisch oder birnformig, zumal 

 in der Nahe der grosseren Blutgefasse. Streckenweise be- 

 halten sie aber ihre friihere abgeplattete Form bei oder 

 nehmen mehr oder wenig an Volumen zu." 



The unsplit mesoderm of the vascular omphalopleure is 

 exactly as in Stage C. 



The ectoderm is, as in that stage, an exceedingly delicate 

 layer of greatly attenuated cells (fig. 24, ect.). In my prepa- 

 rations of this stage, not only is the vascular omphalopleure 

 very evidently stamped with the contour of the highly vascular 

 syncytial surface, but in places the two are in most intimate 

 attachment, thus affording support for the belief already ex- 

 pressed that we have here an actual yolk-sac placental con- 

 nection. 



In this stage, then, we regard the yolk-sac placenta as being 

 in functional activity along with the allantoic, though now it 

 has diminished considerably in importance, as the examination 

 of the foetal circulation shows. The vitelline vein in this stage 

 is both absolutely and relatively smaller than in Stage C, and 

 now most of its blood has to pass through the capillary system 

 of the liver before reaching the inferior vena cava. These 

 facts, taken in conjunction with the already mentioned greater 

 size of the allantoic trunks in this stage as compared with the 



