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Show 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 capillary system of the fcetal 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 foetal tissues in which, excepting the blood, the foetal 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 pro!'ably of fcetal 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 excavated in the trophoblast of the foetus and containing maternal blood interlacing with much branched tufts of foetal capillaries containing foetal 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 foetal surface before undergoing much subdivision and are more central in position. The foetal 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. 288 ON THE FOETUS AND PLACENTA OF THE SPINY MOUSE. [June 6, |
