502 A. A. W. HUBEECHT. 



more intimate^ and so has the confluence of protoplasm by 

 which a syncytial layer of embryonic derivation is created, 

 in which intercommunicating spaces for the maternal blood 

 are gradually evolved. The possibility of distinguishing ma- 

 ternal from embryonic derivates in this layer is yet further 

 diminished ; in the stage of figs. 7 and 49 (uterus No. 42) there 

 are, however, indications that secondarily a deeper layer of the 

 trophoblast becomes more sharply marked off against the 

 syncytial layer, and may thus be compared to what Ed. van 

 Beneden^ has called for the bat the cytoblast, i. e. a deeper 

 layer of the trophoblastic tissue in which the cellular character as 

 distinct from the syncytial is more prominent. Here in Sorex 

 it is only just indicated, and in many sections very imperfectly 

 visible, but certainly somewhat more distinct in later stages. 



Altogether the phenomena here described in the ompha- 

 loidean region of Sorex do not testify of a very intense physio- 

 logical significance of this region. They have only an 

 evanescent existence for the short period that the area vasculosa 

 comes to extend along the belt-shaped zone of the original 

 attachment of the blastocyst. No omphaloidean villi (as in 

 Erinaceus) contribute to the increase of the surface or to a 

 more intimate contact between maternal and embryonic circu- 

 lation. As such the arrangement is perhaps more the here- 

 ditarily transmitted reminiscence of former higher perfection, 

 which has been gradually overruled by the allantoidean 

 arrangement. 



The phase in which the interchange of nutritive materials 

 and of oxygen between the maternal blood and that of the area 

 vasculosa may be said to have reached its maximum is repre- 

 sented in fig. 10 (uterus No. 106). 



In all the sections of this phase I find very marked self- 

 injections of numerous capillary blood-spaces that are imme- 

 diately contiguous with the above-mentioned cytoblastic layer, 

 and only separated by this from the now fully developed circu- 

 lation of the yolk-sac. These blood-spaces originated in the 

 syncytial layer, as was noticed above, and their lumen was 

 'Bulletin de 1' Academic Royale de Belgique ' (3), vol. xv, p. 351. 



1 < 



