288 ON^ THE FCETUS AND PLACENTA OF THE SPINY MOUSE. [June 6, 



which may possibly have contained glycogen — so my evidence on 

 this point is negative. 



Along the line which marks the internal limit of the foetal 

 mesoblast (that is to say, the limit of the capillaiy system of the 

 foetal villi) a deposit of brown pigment occurs. This appeai-s to 

 be deposited in the walls of the capillaries, by the endothelium. 

 It occurs nowhere else. Treated with the ferrocyanide test, it 

 shows no trace of blue colour. 



In the detritus in the layer D (text-fig. 45, p. 285) there are 

 indications of the presence of free iron. 



S'umniary. 



To recapitulate my interpretation of the single specimen I 

 possess. The placenta of Acorays cahirinus is a compound 

 structure of maternal and foetal tissues in which, excepting the 

 blood, the foetal tissue largely preponderates. 



On the maternal side is a centi'al area of attachment through 

 which the maternal blood gains access to the placenta,. Here 

 a thin layer of maternal connective tissue surrounds the main 

 afferent and efferent maternal blood-channels. 



Within this region comes a thick layer of tissue prol:ably of 

 foetal origin (the trophoblast, the cells of which are large, stain 

 deeply, and have large nuclei), containing intercellular spaces, 

 which are continuous with the expanded maternal vessels just 

 named. These spaces are lined by an endothelium, as to the 

 origin of which I can give no account. There is no foetal blood in 

 this part of the placenta. 



These two regions, of which the latter is by far the larger, 

 make up nearly one half of the whole placenta. 



The rest (that is, all towards the foetus) is composed of channels 

 probably excava,ted in the trophoblast of the foetus and containing 

 maternal- blood interlacing with much branched tufts of fcetal 

 capillaries containing fcetal blood. These foetal capillaries are 

 in parts thickly covered with foetal mesoblastic tissue, but more 

 often are separated from the maternal blood by their own 

 endothelium and a single layer of trophoblast only. 



The maternal afferent channels penetrate to the foetal surface 

 before undergoing much subdivision and are more central in 

 position. 



The fcetal afferent vessels tend to penetrate the deeper layers, 

 but begin to break up nearer to the surface of approach than is 

 the case with the maternal afferent vessels. 



There is no such intimate connection between the yolk-sac and 

 allantoic placenta as there is in the Rat. 



