200 



F(ETAL MEMBRANES OF THE RODENTIA. 



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



covered by the mesoblast of 

 the area vasculosa, as indicated 

 by the thick black line {fd). 

 The vascular area is bordered 

 by the sinus terminalis (sf). 

 In an earlier stage of develop- 

 ment the yolk -sack had not the 

 compressed foi'm represented 

 in the figure. It is, however, 

 remarkable that the vascular 

 area never extends over the 

 whole yolk-sack ; but the inner 

 vascular wall of the yolk-sack 

 fuses with the outer, and with 

 the subzonal membrane, and 

 so forms a false chorion, which 

 receives its blood supply from 

 the yolk-sack. This part of 

 the chorion does not develop 

 vascular villi. 



The allantois {al) is a sim- 

 ple vascular sack with a large 

 cavity. Part of its wall is 

 applied to the subzonal mem- 

 brane, and gives rise to the 

 true chorion, from which there 

 project numerous vascular 

 villi. These fit into corre- 

 sponding uterine crypts. It 

 seems probable, from BischofFs 

 and Kolliker's observations, that the subzonal membrnne in the area of 

 the placenta becomes attached to the uterine wall, by means of villi, even 

 before its fusion with the allantois. In the later periods of gestation 

 the intermingling of the maternal and foetal parts of the placenta becomes 

 very close, and the placenta is truly deciduate. The cavity of the allantois 

 persists till birth. Between the yolk-sack, the allantois, and the embryo, 

 there is left a large cavity filled with an albuminous fluid. 



The Hare does not materially differ in the arrangement of its foetal 

 membranes from the Rabbit. 



In the Rat (Mus decumanus) (fig. 149) the sack of the allantois com- 

 pletely atrophies before the close of icetal life', and there is developed, at 

 the junction of the maternal part of the placenta and the unaltered mucous 

 membrane of the uterus, a fold of the mucous membrane which completely 

 encapsules the whole chorion, and forms a separate chamber for it, distinct 

 from the general lumen of the uterus. Folds of this nature, which are 

 specially developed in Man and Apes, are known as a decidua reflexa. 

 The decidua reflexa of the Rat is reduced to extreme tenuity, or even 

 vanishes before the close of gestation. 



Fio. 148. Diagrammatic longitudinal sec- 

 tion OF A Babbit's ovum at an advanced stage of 

 PEEGNANCY. (From Kolliker after Bischoff.) 



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

 tois with blood-vessels ; sli. sub-zonal membrane; 

 j)l. placental villi ; fd. vascular layer of yolk-sack ; 

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

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

 blast lining the compressed cavity of the yolk- 

 sack ; rfs. cavity of yolk-sack ; st. sinus terminalis ; 

 r. space filled with fluid between the amnion, the 

 allantois and the yolk-sack. 



1 This is denied by Nasse; vide Kolliker, No. 183, p. 361. 



