9O J. THOS. PATTERSON. 



tained in experiment III. also do away with the necessity for 

 assuming that the vascular area must start as an " anlage " at the 

 posterior border of the area pellucida. There is no doubt but 

 that the first blood-islands appear here, but this is explained by 

 the fact that the primitive streak mesoderm (from primitive 

 plate) is from the first in close proximity to, if not in direct 

 contact with, the germ-wall at this point. The progressive differ- 

 entiation of the blood-islands from here anteriorly around the 

 inner edge of the area opaca is only an index to the progress 

 made in the expansion of the primitive streak mesoderm. 



