DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 151 



rate from the mass and wander freely on the yolk. The extent 

 of such wandering is probably variable in different species. Yet 

 in all eggs with an extensive vitelline circulation the wandering 

 of these future red blood cells probably takes place to a con- 

 siderable extent in spite of the fact that the cells have been so 

 generally overlooked. A fact easily accounted for when one 

 realizes that most of the studies in blood origin in teleosts have 

 been confined to sections of the embryos of the salmon and 

 trout. Sections are extremely slow in revealing the significance 

 or even existence of wandering cells in development. 



We may now consider the individual wandering cells and their 

 subsequent differentiation. Figure 31 shows a group of six such 

 cells. They are small when compared with the huge chromato- 

 phores but are about as large as the endothelial cells, though 

 completely different in shape, texture and l^ehavior. Figure 32, 

 a camera lucida sketch, serves well to illustrate the relative sizes 

 of the different type cells on the yolk-sac. In this figure are shown 

 all four types, the enormous black chromatophore, the very 

 large brown chromatophore, the delicate elongate endothelial 

 cells of the vascular wall with their filamentous processes, and 

 the almost circular erythroblast, small when compared with the 

 first two types, but as large or even larger than the endothelial 

 cell. 



Figure 34 was drawn from an embryo that had been fixed in 

 such a way as to render conspicuous the cell outlines of the ecto- 

 dermal layer of the yolk-sac. Below the ectoderm groups of 

 erythroblasts forming blood islands are shown, and the extremely 

 small size of the erythroblasts in comparison with the enormous 

 dimensions of the ectodermal cells is most striking. 



As mentioned before, the erythroblasts tend towards a cir- 

 cular shape but send out short processes, while they move in a 

 sluggish amoeboid fashion. The cytoplasm of these cells is 

 slightly greyish and not so perfectly transparent as that of the 

 spindle cells; this difference between the two types is not so 

 marked during the early stages but is readily noticeablo in embryos 

 of 80 or 90 hours. 



The future erythroblasts very slowly wander away from the 

 tail region of the embryo and down the posterior surface of the 



