510 A. A. W. HUBRECHT. 



which in their turn the first origin will have to be settled. 

 For the present I will refrain from giving my own opinion on 

 the point whether these mother-cells are of direct hypoblastic 

 origin, or whether they are proliferations of the ever so much 

 thinner mesohlastic tissue which lines the massive hypoblast 

 of the area vasculosa. 



Lately new researches on the origin of the blood and the 

 blood-vessels (C. K. HoflFmann, a. o.) have brought this ques- 

 tion again very much into the foreground, and rather than 

 here treating it incidentally I will limit myself to the pointing 

 out of the shrew's yolk circulation as a favorable object for 

 the study of these problems. 



Figs. 64 and 65, taken from the same section, show two 

 blood-vessels of the yolk-sac in a much later stage. The upper 

 one is a very much flattened space in a stretched portion of the 

 area vasculosa ; the lower one is in the region close to the free 

 border of the placenta (cf. fig. 15 a), where the surface of the 

 area vasculosa is thrown into very numerous folds, the free 

 space in the yolk-sac being in these later phases of pregnancy 

 more and more reduced. The coagulum is all the same pre- 

 sent in these later stages. The hypoblast cells are here seen to 

 have yet further increased in size. 



An important peculiarity that should here be mentioned, 

 now that the blood-corpuscles of the embryo and their neo- 

 formation are being noticed, is this, that they are of such a 

 different size from those of the mother. Such is the case, not 

 only in the early (figs. 80—82, 85, 86, 89), but also in the later 

 stages of pregnancy, and off"ers a most valuable advantage for 

 recognising maternal from embryonic circulatory spaces. This 

 is especially important in the placental region, where the rela- 

 tive intermixture of these spaces is so extremely complicated, 

 and where quite normal self-injections are thus available, 

 showing the finest blood-spaces with the utmost clearness (figs. 

 52 and 53). 



We have now finished the description of the later pheno- 

 mena in the omphaloidean region and the area vasculosa, in 

 that of the trophoblastic annulus, and of the non-placental 



