; 



150 



disappears, and that the space which it originally filled is reoc- 

 cupied by other glycogenic cells which are of embryonic origin. 

 A discussion of the relative rnerits of this and of other possible 

 views will be given later on. 



In the allantoic region the counective tissue covering the smaller 

 villi disappears, haviug been used up, I irnagine, in the form- 

 ation of new capillaries; at the same tirne the en dotheli urn lining 

 these capillaries becomes exceedingly thin (Fig. 37). In this period 

 also the non-nucleated corpuscles appear in the foetal circulation. 

 The trophoblast covering the capillaries persists as a tbin layer, 

 except where the nuclei cause it to project into the lacunae; it 

 is, on the whole, tbicker than the endothelium of the capillaries; 

 it never entirely disappears, and raay still be detected in the 

 actual afterbirth. 



In the last period we left the region of the trophoblast which is 

 not penetrated by the allantois as a layer of glycogenic cells covering 

 the raaternal surface of the placenta. The uuder surface of this 

 layer is now as before irregular; portions of it in which cell 

 division is still going on, and in which vacuolation is only just 

 commencing may be found dipping down into the allantoic region 

 of the trophoblast. 



Further, there now appears between this layer and the outer 

 limit of the allantoic region a series of trophoblastic cavities. 

 These cavities are found extending over a broad annular zone, 

 which corresponds with the area occupied by the megalokaryo- 

 cytes; they do not themselves contain blood but lie between 

 peripheral channels by which the lacunae empty their blood into 

 the venous sinuses. 



They are irregular in shape, lined by large branched tropho- 

 blastic cells, projecting into the interior. In the interior a coa- 

 gulum may be observed and in addition minute granules, which 

 seem to be liberated by the degen eration of the trophoblastic 

 cells which once occupied the cavity; just as, as we shall shortly 

 see, similar granules are dispersed by the breaking up of the 

 maternal glycogen cells. In the latest stages of gestation the cells 



