60 CHARLES R. STOCKARD 



The mesenchymal cell with the power of formmg vascular en- 

 dothelium is probably very diffusely distributed throughout 

 the embryonic body as well as the yolk-sac. Numerous investi- 

 gators have supplied evidence indicating this fact. On the 

 other hand, the mesenchymal cells which are to form the erythro- 

 blasts are in the bony fish very definitely localized. The latter 

 cells are chiefly confined to the intermediate cell mass, but in 

 addition other erythroblast forming cells wander out probably 

 from the same source of origin, the primary intermediate cell 

 mass, to become distributed on the yolk-sac. 



Finally, the mesenchymal cells which are to give rise to lym- 

 phocytes and leucocytes of various types seem in Fundulus 

 embryos to be more or less localized in the head and anterior 

 region of the body and do not seem to be particularly associated 

 with vessels. For this reason in early embryos the first apparent 

 lymphocytes and leucocytes are found in the head and anterior 

 body regions, and even in older individuals such cells are more 

 abundant here than in other portions of the body. Yet these 

 cells are doubtless of a roving or wandering type and may finally 

 become scattered throughout the embryo's body. While the 

 non-migrating red blood corpuscles rarely if ever leave their 

 original sites of differentiation. 



7. Environmental conditions necessary for blood cell multiplication 



and differentiation 



The above facts and interpretations lead us to a consideration 

 of the later conditions of cell multiplication and differentiation. 

 Are the so-called haematopoetic organs of the embryo and 

 even the adult actually haematopoetic, or are they merely favor- 

 able localized environments in which various types of blood cells 

 may multiply or reproduce themselves throughout the life of the 

 embryo or individual? There is little doubt, from the recent 

 suggestive studies on the cultivation of tissues in artificial media 

 out of the body of the organism, that certain environmental 

 surroundings are conducive to cell proliferation and growth 

 while other environments inhibit these processes and tend to 

 favor differentiation and functional activity. 



