44 CHARLES R. STOCKARD 



tral tissue mass in a living tissue culture. The cells are elongated 

 spindle-form and all are moving straight away from their seat 

 of central origin. This phenomenon is well illustrated by the 

 numerous figures of tissues growing in culture media and I shall 

 give illustrations of it in a special study of this subject now in 

 preparation. 



In all of the non-pelagic bony fish eggs investigated up to 

 now, the chief blood forming cells are without exception the in- 

 tra-embryonic intermediate cell mass, and this mass is claimed 

 to form both vessels and blood. While in the pelagic type of 

 bony fish egg the mass is usually concerned in the formation of 

 vascular endothelium, and the blood cells only arise after the 

 embryo is hatched and free swimming. 



This peculiar specialization in intra-embryonic blood forma- 

 tion which seems typical for the bony fish has caused the yolk- 

 sac formation of blood to be almost completely neglected or 

 overlooked by recent investigators. Yet in the species upon 

 which I have experimented there is no doubt whatever that 

 blood islands do arise on the yolk and their origin is from the 

 wandering mesenchymal cells. The wandering cells may be 

 connected in some manner with the intermediate cell mass, yet the 

 presence of the islands cannot be explained in the way Swaen 

 and Brachet ('01) have attempted to account for the yolk-sac 

 blood. They assume that the islands are pushed out laterally as 

 branches or portions of the intermediate cell mass. In many cases 

 no direct continuation of cells is traceable between the yolk 

 islands and the intermediate cell mass and even in extremely 

 young embryos yolk islands may appear on the ventral yolk sur- 

 face at a great distance away from the intermediate cell mass. 



The group of four figures, 36 to 39, indicate the progressive 

 patterns assumed in life by these yolk islands. In the very early 

 stage, figure 36 shows separate collections of cells here indicated 

 by stippling. These groups then become confluent as in figure 

 37, then more or less net-like in appearance with certain nodes 

 or portions thicker and darker than the general net. In these 

 nodes cell proliferation or blood formation is more active. Finally 

 a typical vascular network arises which goes to make up the cap- 

 illary yolk circulation of the embryo. 



