92 CHARLES R. STOCKARD 



This is all the diphyletic or polyphletic school would ask. 

 That is, that certain definite mesenchymal cells are actually 

 the red blood cell anlage and only from these particular mesenchy- 

 mal cells do red blood cells arise. Here we logically stop, for 

 this is what is conceived by embryologists to be an anlage, back 

 of this we go to certain germ layers and still further back to the 

 developmental potentialities of certain individual cells as fol- 

 lowed in studies of cell-lineage and finally we reach the elemen- 

 tary proposition of deriving everything from the original egg cell. 

 But to stop with the tissue anlage we find strong evidence to 

 indicate that certain special mesenchymal cells are designated 

 to form erythroblasts, others leucoblasts, and still others, and 

 these are much more universally scattered throughout the 

 embryo, give rise to vascular endothelium. These latter may 

 really be admitted to form endothelium largely as a response 

 to physical conditions. 



SUMMARY AND CONCLUSIONS 



The present contribution attempts an experimental analysis 

 of the origin and development of blood cells and the endothelial 

 lining cells of the vascular system. Studies on the origin of 

 blood and endothelium in the normal embryo are rendered 

 peculiarly difficult on account of the important role that wander- 

 ing mesenchyme cells play in this process as well as the perplexing 

 mixture of cells of different origin brought about by the early 

 established circulation. The origin of no other tissue is so con- 

 fused by mechanical and physical conditions. 



The first difficulty has been met by a study of living Fundulus 

 heteroclitus embryos with the high power binocular microscope. 

 The wandering mesenchyme cells may in this way be followed 

 to a great extent. The disadvantages due to the intermixture of 

 cells in the blood current have been overcome by the investi- 

 gation of embryos in which a circulation of blood is prevented 

 from taking place. 



When Fundulus eggs are treated during early developmental 

 stages with weak solutions of alcohol the resulting embryos in 



