STUDIES IN MAMMALIAN EMBRYOLOGY. 509 



secoud instance of food material for the growth of the embryo, 

 this material being at the same time partially derived from 

 other sources. 



We have now to give a somewhat fuller account of the 

 changes in the area vasculosa that have been so often referred 

 to. The most evident change relates to the hypoblast cells of 

 this region, w^hich become more bulky and which take a most 

 vivid green colouring, visible through the distended tissue of 

 the unopened living uterus. This colouring matter is extracted 

 by alcohol. It was not further chemically analysed, but after 

 the ordinary treatment of the specimens with hardening, stain- 

 ing, and embedding reagents, it can yet be distinguished as a 

 greenish-brown or reddish-brown colour. The increase in the 

 size of the hypoblast cells can already be detected when the 

 stage of fig. 12 is reached. From that of fig. 14 onwards it 

 is, however, most clearly marked, and at the same time another 

 process comes in the foreground in this region, viz. the forma- 

 tion of embryonic blood-corpuscles. Different stages in the 

 formation of those corpuscles are represented in figs. 58 — 63, 

 and I have no doubt that the preparations there figured 

 will well repay a conscientious study of this process. As the 

 dimensions of the vascular surface of the yolk-sac increase so 

 considerably in the period from fig. 13 onwards, new vessels 

 are being formed in all directions, and become visible as pro- 

 liferations that rise above the level of the surface (figs. 58 and 

 59) and gradually form a raised network with the pre-existing 

 larger vessels. The whole of this network is bathed, as was 

 noticed above, by the contents of the yolk-sac. In these 

 vessels the future lumen is filled with cells which in the first 

 instance are fixed and immoveable (fig. 58, extreme right), each 

 of thera being granular and only gradually changing their 

 aspect — transition stages being present — in that of the ordinary 

 embryonic blood-corpuscles. At the same time the latter are 

 seen to become looser (fig. 58, middle), and are gradually 

 attracted into the general embryonic circulation. 



As to the first origin of these cells, it can be noticed that 

 they arise out of larger polynuclear ones (figs. 59 — 63), of 



