486 A. A. W. HUBREOHT. 



This syncytium owes its origiu to that portion of the surface 

 of the blastocyst which we will call the omphaloidean tro- 

 phoblast. 



2. A bell-shaped syncytium opposite the mesometrium. 

 This syncytium owes its origin to that part of the outer wall 

 of the blastocyst which expands simultaneously with the 

 formation of the amnion (being, in fact, the epiblast of the 

 outer amnion fold), and which we will call the allantoidean 

 trophoblast. 



Both syncytia enclose numerous cavities, into which maternal 

 blood penetrates. 



The omphaloidean syncytium with the area vasculosa applied 

 against it is not indented by villi of the yolk-sac, such as are 

 found in the hedgehog. The area vasculosa is after some 

 time removed, the regions agaiiist which it has been applied 

 (proliferation and syncytium) being gradually but entirely 

 resorbed. A new layer of maternal uterus-epithelium applied 

 against the muscularis arises directly below the resorbed 

 portions. 



When this has come about the bell-shaped syncytium oppo- 

 site the mesometrium remains alone in the field, and undergoes 

 a series of further transformations and complications, which 

 change it into the full-grown discoid placenta of the shrew. 



These transformations can be summarised as follows : 



(a) The allantoidean trophoblast is applied against the concave 

 maternal surface, and sends knob-like projections into the 

 mouths of the maternal crypts, the maternal epithelium being 

 destroyed wherever the trophoblast adheres. The projections 

 serve to fix the trophoblast very firmly against the maternal 

 proliferation. They do not enter the mouths of uterine glands ; 

 these are simply blocked by the trophoblast. 



(6) The trophoblast undergoes a differentiation into an outer 

 layer, which assumes the syncytial character more fully, and 

 contains paler nuclei (plasmodiblast — van Beneden), and an 

 inner layer of which the nuclei stain more strongly (cytoblast — 

 v. Beneden). 



(c) Internuclear blood-spaces are developed in the plasmodi- 



