AND THOSE OF CERTAIN OTHER MAMMALIA. 91 



media in any placenta, as that of the guinea-pig, which I have 

 been able to manipulate. Neither have I been so fortunate as 

 to find this system of tags in the common rat (Mus decumanus) ; 

 and but that the homologous structures, two only in number, 

 which are to be seen in the rabbit (Lepus cuniculus), are far from 

 being invariable in their appearance, I should suggest that the 

 foetal rat in question was of the black rat [Mus rattus) species. 



The persistent or non-deciduous serotina in the guinea-pig is 

 inseparable, save to the eye aided by the microscope, from the 

 circular muscular coat. 



Bischoff 1 says of the decidua serotina, that it separates from the 

 regenerating mucous coat immediately after the separation of the 

 placenta, and is either discharged or, as he thinks, in many cases 

 quickly absorbed ; and Reichert 2 believes that certain structures 

 found not rarely in the uteri of pregnant guinea-pigs may be the 

 remnants of the placenta uteri na and decidua reflexa of previous 

 pregnancies. I have no observations of my own with reference to 

 what takes place in the natural order of things, but in the after- 

 births of guinea-pigs killed at full time I have found the deciduous 

 serotina in some cases so firmly cohering with the placenta and 

 with the upgrowth of it as to make it difficult to believe that 

 it does not occasionally come away with it in parturition, as by 

 actual observation we know it does in the rat. Variations may 

 occur in the case of the former animal. 



The guinea-pig and rat resemble each other in having, in early 

 stages of development, a perfect decidua, in having their chorion 

 attached to the centre of a unilobed placenta, and in having the sac 

 of the allantois early obliterated ; and in all these points they differ 

 from the Leporidae. 



The circlet of vascular villi lying exteriorly to the placental area, 

 and supplied exclusively by the omphalo-mesenteric system, is said 

 by Bischoff 3 to spring up only in the latter part of foetal life. 

 In those rodents in which, as in the Muridae, and in the guinea- 

 pig, the chorion is attached by an apex, as it were, to the centre 

 point of the floor of the placenta, we see, on making a transverse 

 section of a uterine dilatation containing an advanced foetus, that 

 a considerable interval exists on either side of the attachment of 



1 'Meerschweinchen-Ei,' p. 44. 2 ■ Beitrage,' p. 131. 



3 ' Meerschweinchen-Ei,' pp. 43 and 44. 



