( 615 ) 



is found to exhibit soniewluit ditrereut conditions. In dogs, for 

 instance, according to the investigations of Bischoff '), Strahl'*) and 

 others, the allantois-sac forces its wny between the chorion and the 

 amnion. While the ailantois is thus making its way further and 

 further between the two membranes, gradually separating them 

 almost entirely, the edges of the ailantois are coming closer together. 

 But these edges, according to the drawings of Bischoff (I.e. Plate XV, 

 fig. 8) remain separated; according to Strahl^), they establish contact 

 by means of cellular tissue, chorion and amnion being thus connected 

 at this point of contact. In Phoca I found neither the edges of the 

 allantois-sac, nor a comiection like Strahl noticed in dogs; I must 

 therefore assume that in Phoca the development has gone a little 

 further and that through the elimination of the partition between 

 the two edges of the allantois-sac, mentioned by Strahl, the outer 

 and inner sac have become entirely isolated. The Avhole of the 

 original extra-embryonal coelom-cavity has in this way disappeared; 

 the space between the inner and outer foetal sac is the allantois-cavity. 



I have already pointed out that the umbilical cord was connected 

 with the placenta by means of a so-called insertio velamentosa. The 

 vessels running towards the placenta or in the inverse direction, are 

 enclosed by the membrane covering the foetal side of the placenta, 

 viz. the outer layer of the allantois-sac, which continues on the 

 chorion along the edge of the placenta. 



As far as the inner foetal membrane is vascularised, the vessels 

 show the same proportions as in the outer foetal sac. 



The umbilical sac is situated in the inner wall of the foetal sac, 

 between tiie amnion and the ailantois, connected by thin cords of 

 cellular tissue with both. It is a much-elongated, narrow bag, with 

 folded walls. 



The walls consist of strongly vascularized connective tissue, its 

 limits are not sharply separated from the stroma of the amnion- 

 and ailantois membranes. Oidy here and there Ave find the remains 

 of an original coating of epithelium. The bloodvessels, some of which 

 have a fairly large lumen, run almost en tii-ely lengtinvays through the 

 organ. The majority of these vessels have a thick muscular wall and 

 were filled with blood. These vessels and tije umbilical ones are 

 connected with one another. 



With regard to the placenta the following may here be stated. 



1) Bischoff, Die Entwickelungsgeschichte des Hundeëies. 



2) Strahl, Untersuchungen über den Bau des Placenta. III. Archiv fur Anatomie 

 und Entwickelungsgeschichte. 1890. 



S) I.e. p. 199. 



41 



Proceedings Royal Acad. Amsteidam. Vol. VI. 



