DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 145 



When the circulation begins, however, this cell becomes loosened 

 away from the wall for mechanical reasons, the lack of long 

 processes, etc., and projects into the lumen to be finally washed 

 away. Any one may readily observe such occurrences who 

 will study the living yolk-sac of Fundulus with a high power 

 microscope and a strong condenser so as to use a darkened field. 



All of these observations lead one to conclude that the only 

 connection between vascular endothelium and primitive blood 

 cells is one of association. The endothelial cells never meta- 

 morphose into blood cells. It is important here to recall the 

 fact previously emphasized by the author that in those speci- 

 mens in which there is never a circulation of the blood or plasma 

 the vascular endothelium develops in a perfectly normal manner 

 in the aorta and other intra-embryonic vessels, as well as in the 

 vessels on the yolk-sac, yet in none of these does one find any 

 appearance indicating a tendency of the lining cells of the vessel 

 wall to change into any type of blood cell. Numerous other 

 details from my notes might be enumerated which would bear 

 on this question, but sufficient care has been taken to definitely 

 establish the above crucial points as facts. This may not of 

 course hold for all types of animals but it does for those studied. 



I have seen a number of sections on which other investigators 

 have based their claim that vascular endothelial cells do change 

 into primitive blood cells and although inclined towards the 

 acceptance of such a view from a mere acquaintance with the 

 literature, a study of such material has convinced me that the 

 negative interpretation is equally plausible in all cases. 



3. Blood corpuscles on the yolk-sac of teleost embryos 



In all meroblastic eggs except those of the teleost a great 

 sheet of mesoblast is found extending over the yolk as the so- 

 called peripheral or ventral mesoderm, or subvitelline mesoderm. 

 It is this peripheral mesoderm that gives rise to the blood islands 

 of the yolk-sac in selachians, reptiles, birds and mammals. The 

 teleost, however, presents a unique case in that the ventral 

 mesoderm does not spread out over the yolk but is included 



