288 ON THE FG@TUS 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 fcetal 
mesoblast (that is to say, the limit of the capillary system of the 
foetal villi) a deposit of brown pigment occurs. This appears 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. 
Summary. 
To recapitulate my interpretation of the single specimen I 
possess. The placenta of Acomys cahirinus is a compound 
structure of maternal and feetal tissues in which, excepting the 
blood, the feetal tissue largely preponderates. 
On the maternal side is a central 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 probably of 
feetal 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 noaccount. There is no feetal 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 excavated in the trophoblast of the foetus and containing 
maternal blood interlacing with much branched tufts of feetal 
capillaries containing fcetal blood. These foetal capillaries are 
in parts thickly covered with fcetal 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 feetal surface 
before undergoing much subdivision and are more central in 
position. 
The fcotal 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, 
