Foetal Membranes of the American Beaver (Castor canadensis). 



203 



in an equal manner at each end. The opening of each inter-utricular 

 junction lies within this area (Fig. E). 



Corresponding with the uterine pyriform areas, which we may 

 distinguish as the utero-placental areas, there are two simi- 

 larly shaped transparent Windows or fenestrae pyriform es, in 

 other words, the umbilico-placental areas, in the roof of the 

 chorion. The latter areas are, in general, non-vascnlar, bnt occasionally 

 a very iine vessel may be seen, here and there, traversing the mem- 

 brane for a short distance and entering the sinus terminalis which 

 borders it. This may indicate the existence, at an earlier stage, of 

 a more thorough vascnlarisation of this part of the chorion and, if 



Fig. F. 



Diagrarn of tlie placental aspect of the detached blastocyst. Only the main omphalo- 



meseuteric vessels are indicated. 

 1. Stalk of placenta. 2. Fenestrae puriformes (umbilico-placental areas). 3. Sinus 



terminalis. 4. Artery. 5. Vein. 



