406 .TAS. p. HILL. 



tinuous with the somatic mesoderm of the chorion, forming 

 the allanto-chorionic mesenchyme. This is, at this stage, an 

 exceedingly thin layer, which becomes somewhat thicker at the 

 margin of the allantoic placental area and around the larger 

 allantoic capillaries (figs. 6, 7, and 8, all. mes.). The mesen- 

 chyme consists of small branched cells, the delicate processes 

 of which anastomose with each other, and with the entodermal 

 lining of the allantoic cavity. I find that the allantois of a 

 late uterine embryo of Macropus dorsalis shows essentially 

 the same structural features as the vesicular portion of the 

 allantois of Perameles, only in the Macropus embryo the 

 vessels are less marked ; and there is, of course, no union with 

 the chorion. In both cases the allantoic wall is characterised 

 by its extreme tenuity. 



In the inner or ccelomic wall of the allantois, between the 

 mesothelium and the entoderm, run the main branches and 

 factors of the allantoic arteries and vein. Corresponding to 

 their characteristic mode of branching, one finds in sections 

 the larger trunks in pairs, a smaller and slightly thicker 

 walled arterial trunk accompanied by a larger venous channel. 

 In the inner wall the vessels have distinct thin sheaths of 

 condensed mesenchyme. In order to reach the outer surface 

 of the allantois these vessels, ramifying in the inner wall, turn 

 round the periphery of the flattened vesicle. Seeing the 

 allantoic cavity is a continuous and uninterrupted one, there 

 are no direct passages across the cavity by means of cellular 

 bridges, as Hubrecht describes for the allantois of Erinaceus 

 (9). The vessels of the inner wall gradually decrease in size 

 by repeated branchings as they approach the periphery of the 

 vesicle, their mesodermal investing sheath becomes reduced to 

 a layer a single cell thick, and they pass round into the 

 allanto-chorionic mesenchyme. There they again branch re- 

 peatedly, forming a network of small capillaries, with only 

 endothelial walls. The ultimate branches of this capillary 

 system come into very close contact with the inner surface of 

 the chorionic ectoderm, and even in places where cells of the 

 latter have disappeared, into contact with the vascular maternal 



