AND THOSE OF CERTAIN OTHER MAMMALIA. 107 



which comes into relation and connexion with the placenta gives 

 way in the rodent, and allows the cuplike mass which it attaches 

 to the uterine wall to drop freely into the uterine cavity; if it 

 gives way in the ruminant, as it sometimes does, it is a patho- 

 logical process which entails, pro tanto, sterility upon the animal 

 in which it occurs, and has its occurrence marked by the production 

 of a cicatrix. In other words, the fibro-plastic cells which may 

 be found on the utero-placental area of a ruminant, from which 

 the above downgrowth proceeds, are not sufficient for the regene- 

 ration of the mucous structures over that area, which perfect 

 regeneration however we do find to take place in the whole class 

 of deciduate mammals. Nor is this all. For though the placenta 

 with its upgrowth (Kern of Bischoff) in the guinea-pig might 

 be taken to represent, though roughly, the cotyledons on the dis- 

 charged placenta of a ruminant, the structures which they were 

 drawn from in the rodent differ essentially from the placentulae 

 of the ruminants, in that they contain inextricably mixed up in 

 their mass, as a well-injected specimen will always show, maternal 

 elements which the apparently similar ruminant structures do not l . 

 In its early attainment of the faculty of self-help, however, in the 

 inguinal position, and in the small number of its mammae, the 

 guinea-pig presents points of real resemblance to the ruminants 

 and also to most other non-deciduate mammals. 



Secondly, Professor Owen, in the Linnean Society's 'Proceedings 2 ,' 

 says of the deciduous portion of the rat's placenta, that it — 



* Consists of foetal parts exclusively ;' and that the c structure of the discoid pla- 

 centa in the Pteropus, like that of the rat, more resembles that of the foetal portion 

 of the cotyledon in the cow than that of the cellulo-vascular, spongy placenta of the 

 Quadrumana.' 



To this it must be replied that specimens, such as most museums 

 possess, of uteri containing foetuses in which both foetal and 

 maternal vessels have been injected show distinctly that this 

 resemblance does not exist. It is impossible to inject the uterine 

 vessels of any deciduate mammal, at any but the very earliest 



1 It should be borne in mind that, though in the pregnant uteri of these prolific 

 animals the sites of former placentae are recognisable as well as the functional struc- 

 tures, the sites of the two hardly ever coincide. Reichert says that he has only once 

 seen the new placenta attach itself to the place occupied by its predecessor (1. c. 

 p. 130). Matters are altogether different in the ruminants. 



2 1. c. p. 16, note. See also ' Phil. Trans.' for 1857, p. 351. 



