204 Abthur Willey, 



so. it would correspond with the condition described by Duval in 

 the rabbit, where a temporary anastomosis between the allantoic and 

 omphalo-mesenteric Systems takes place through the somatic mesoderm 

 of the umbilico-placental zone, about the 14 th day of gestatioii. 



The mesometric side forms the upper side or roof a foetal 

 utricle. Therefore the inter-utricular segments open at each end of 

 the utricle upon the roof of an otherwise closed space, the utero- 

 placental-space, the floor of which is formed by the umbilico- 

 placental membrane of the chorion, the boundary-walls by the 

 umbilico-uterine ligament, the narrow end being occupied by the 

 extremity of the placenta (Fig. E). 



The general aspect of the superior or placental wall of the 

 blastocyst, after detaching it from its adhesions to the mesometric 

 wall of the uterus is shown in Fig. F. The placenta projects wholly 

 within the chorion, the latter being inserted round the hilus on the 

 upper or mesometric aspect; so that the upper hemisphere of the 

 blastocyst presents a relatively even surface, except for its villosities 

 and the toin disc of attachment to the decidua serotina or 

 „maternal placenta". I have not examined the circulation in the 

 area vasculosa by the aid of injection, but am able to say that 

 there is a good deal of Variation in the ramifications of the main 

 trunks. The sinus terminalis does not in every case form an 

 uninterrupted ring round the disc of attachment of the placenta, but 

 may receive branches at diiferent points, looping partially round the 

 disc from those points to form an interrupted ring. The pyriform 

 areas are constant. 



In the female which contained two foetus it was noted that the 

 lower one presented its caudal end towards the vagina, while the 

 upper one presented the head. The uterine Chambers contained a 

 brown fluid. 



5. Chorion and Placenta. 



The macroscopic relations of chorion and placenta in the beaver 

 at the period of maturity will be better understood by comparing 

 them with the corresponding parts in the guinea-pig, taking as 

 the basis of the comparison Düval's, PI. XVII, Fig. 269 (1892) or 

 Tafani's, PI. IV, Fig. 1 (1886) reproduced in Düval's text-figure 

 XCVIII (1892, p. 424); and in the rabbit as shown in Düval's text- 

 figure CIV A and B (1892, p. 446) which interprets the relations 



