Foetal Membranes of the American Beaver (Castor canadensis). 199 



pressure from within, and would rupture. The closure of the passages 

 is effected by approximation of the uterine walls assisted by the 

 horns of the blastocyst which act as plugs. There is, however, no 

 concrescence between the horns of the blastocyst and the uterine 

 wall in the rabbit. It appears that they persist to the end of 

 gestation, inasmuch as the mature blastocyst of the rabbit does 

 not lie sharply circumscribed within its utricle. but the chorion is 

 prolonged into the adjoining cavity of the uterus. 



Fig. B. 



Fisr. C. 



Fig. B. Diagram of a uterus of the beaver showing four utricles and the 

 placento-uterine membranes stretching across the inter-utricular passages. 1. Sinus 

 terminalis. 2. Inter-utricular segment. 3. Placento-uterine membraue. 4. Placenta. 

 5. Chorion or wall of blastocyst. 



Fig. C. Semidiagrammatic sketch of the fresh blastocyst as seen after removal 

 of the autimesometric wall of the utricle. The branching blood-vessels are the 

 main trunks of the area vasculosa which traverse the substance of the chorion. 

 1. Point where the omphalo-mesenteric vessels enter the chorion. Just below the 

 index-line is seen the opening of the omphalo-mesenteric caual at the surface of 

 the chorion. 2. Umbilico-uterine membraue. 3. Uterine wall. 4. Placenta (seen 

 through the chorion). 5. Position of stalk of placenta. 6. Inter-utricular passages 

 cut across. 



