242 FCETAL MEMBRANES OF THE RODENTIA. 



the true chorion, from which there project numerous vascular villi. These 

 fit into corresponding uterine crypts. It seems probable, from Bischoff' s 

 and Kolliker's observations, that the subzonal membrane 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 fcetal 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 fcetal 

 membranes from the Rabbit. 



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

 pletely atrophies before the close of fcetal life 1 , and there is developed, at 



FIG. 149. SECTION THROUGH THE PLACENTA AND ADJACENT PARTS OF A RAT 



ONE INCH AND A QUARTER LONG. (From Huxley.) 



a. uterine vein ; b. uterine wall ; c. cavernous portion of uterine wall ; d. deciduous 

 portion of uterus with cavernous structure ; i. large vein passing to the foetal portion of 

 the placenta ; f. false chorion supplied by vitelline vessels ; k. vitelline vessel ; /. 

 allantoic vessel; g. boundary of true placenta; e, m, m, e. line of junction of the 

 deciduate and non-deciduate parts of the uterine wall. 



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. 



Guinea-pig. The development of the Guinea-pig is dealt with else- 

 where, but, so far as its peculiarities permit a comparison with the Rabbit, 

 the agreement between the two types appears to be fairly close. 



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



