MAMMALIA. 



241 



vascular yolk-sack fusing with the remainder of the subzonal membrane. 

 In all the existing forms with this arrangement of foetal membranes, the 

 placenta is deciduate. This, however, was probably not the case in more 

 primitive forms from which these are descended 1 . The placenta would 

 appear from Ercolani's description to be simpler in the mole (Talpa) than 

 in other species. The Insectivora, Cheiroptera, and Rodentia are the 

 groups with this type of placenta ; and since the rabbit, amongst the latter, 

 has been more fully worked out than other species, we may take it first. 



The Rabbit. 1 the pregnant female Rabbit several ova are gene- 

 rally found in each horn of the uterus. The general condition of the egg- 

 membranes at the time of their full development is shewn in fig. 148. 



The embryo is surrounded by the amnion, which is comparatively small. 

 The^yolk-sack (ds~) is large and attached to the embryo by a long stalk. 

 It has the form of a flattened sack closely applied to about two-thirds of the 

 surface of the subzonal membrane. The outer wall of this sack, adjoining 

 the subzonal membrane, is formed of hypoblast only ; but the inner wall is 

 covered by the mesoblast of 

 the area vasculosa, as indi- 

 cated by the thick black line 

 (fd}. The vascular area is 

 bordered by the sinus ter- 

 minalis (sf). In an earlier 

 stage of development the 

 yolk-sack had not the com- 

 pressed form represented in 

 the figure. It is, however, 

 remarkable that the vascular 

 area never extends over the 

 whole yolk-sack ; but the in- 

 ner vascular wall of the yolk- 

 sack fuses with the outer, 

 and with the subzonal mem- 

 brane, and so forms a false 

 chorion, which receives its 

 blood supply from the yolk- 

 sack. This part of the cho- 

 rion does not develop vas- 

 cular villi. 



The allantois (al) is a 

 simple vascular sack with a 

 large cavity. Part of its wall 

 is applied to the subzonal 

 membrane, and gives rise to 



1 Vide Ercolani, No. 197, and Harting, No. 201, and also Von Baer, Entivick- 

 lungsgeschichte table on p. 225, part I., where the importance of the limited area of 

 attachment of the allantois as compared with the yolk-sack is distinctly recognised. 



B. III. l6 



FIG. 148. DIAGRAMMATIC LONGITUDINAL SEC- 

 TION OF A RABBIT'S OVUM AT AN ADVANCED STAGE 

 OF PREGNANCY. (From Kolliker after Bischoff.) 



e. embryo ; a. amnion ; a. urachus ; al. allan- 

 tois with blood-vessels; s/i. subzonal membrane; 

 //. placental villi ; fd. vascular layer of yolk-sack ; 

 ed. hypoblastic layer of yolk-sack; ed' . inner por- 

 tion of hypoblast, and ed" . outer portion of hypo- 

 blast lining the compressed cavity of the yolk- 

 sack ; ds. cavity of yolk-sack ; sf. sinus terminalis ; 

 r. space filled with fluid between the amnion, the 

 allantois and the yolk-sack. 



