Foetal Membranes of the American Beaver (Castor canadensis). 205 



between the endoderm of the umbilical vesicle and the ectoplacenta 

 of the rabbit, including the intervening umbilico-placental zone. 



The guinea-pig and still more the agouti (see Duval, 1892. 

 p. 362, footnote) have a stalked placenta, and so to a certain extent 

 has the beaver, although here the stalk is buried in the superior 

 hilus of the placenta. In the guinea-pig and in Rodents generali}', 

 the placenta shows an inferior or foetal hilus where the allanto- 

 chorionic mesoderm enters the ectoplacenta; and a superior hilus 

 marked by the entrance of the utero-placental vessels. 



The mature placenta of the guinea-pig has the form of a circu- 

 lar disc, appearing as an ellipse in vertical section with the trans- 

 verse diameter nearly three times the length of the vertical. On 

 the upper side of this disc, the central third of it is occupied by 

 the placental stalk, the border of which constitutes the zone of 

 adhesion of the placenta to the serotina. Connected with this zone 

 is the residuum of the decidua Capsula ris, forming a residual 

 zone different from that so-named by Duval in the rabbit's blasto- 

 cyst. The rest of the upper side of the disc consists of the free 

 upper surface of the placenta, separated from the uterine wall by 

 the utero-placental groove or recess. On the lower side of the 

 placental disc, the central quarter of its area is occupied by the 

 entrance of the allantoic stalk, at the base of which the amnion is 

 inserted; and between this point and the free edge of the disc, is 

 the zone of insertion of the vascular chorion (or proximal wall of 

 the umbilical vesicle) into the inferior or foetal surface of the pla- 

 centa. This in fact is the umbilico-placental insertion of the guinea- 

 pig, the placental stalk forming the sole connection between blasto- 

 cyst and uterus. 



The chorion of the beaver is inserted at the upper side of the 

 placenta round the placental stalk ; and from there the insertion is 

 continued into the uterine wall by the mediation of the umbilico- 

 uterine ligament round the above mentioned pyriform areas. Outside 

 the umbilico-placental insertion, but still on the upper surlace, is 

 the line of insertion of the amnion, i e. the amnio-placent al 

 insertion. circumscribing an oval or elliptical area. The level 

 of this line varies in different specimens ; it is sometimes mach lower 

 down (i. e. towards the foetal aspect) on the peripheral surface of 

 the placenta (Fig. G). 



The outer surface of the chorion is beset with close-set endo- 

 dermic villi which have the form of linear, more or less undulating 



