168 CHARLES R. STOCKARD 



to its ultimate end. The primordial mesoderm cell or cells 

 carry within their bodies all the potentialities of the mesoderm 

 and may give rise to a series of cells which are capable of develop- 

 ing muscle, cartilage, bone, connective tissue proper, blood 

 cells, vessels, etc. Yet after a few cell generations the indi- 

 viduals in the series derived from these early cells containing all 

 the mesodermal potentialities no doubt become somewhat limited 

 as to their potentialities. In a certain generation there may be 

 definite cells more or less generally distributed which possess 

 the capacity to give rise to muscle cells, but to no other type of 

 mesodermal tissues. Still later in development these cells may 

 come to be even more limited in their developmental capacities 

 and thus have the power to produce only a certain type of muscle 

 cell and no other type. 



Collections of such cells would then be designated embryo- 

 logically as the anlage of striated muscle, smooth muscle, or 

 heart muscle as the case might be Yet it is not to be forgotten 

 that at this stage there might be really no means of distinguish- 

 ing between the several different types of mesodermal cells. 



Limitization of potentialities in the individual cells has ap- 

 parently reached a comparable stage just about the time when the 

 mesenchymal cells begin to wander upon the yolk-sac of Fundulus. 

 We have seen these cells as they wander out and have noted how 

 very soon they may be separated into four distinctly different 

 types, and following the development and behavior of these 

 types it has seemed evident that they are entirely separate and 

 do not intergrade or transmutate. The black chromatophore 

 does not change its nature or divide off other cells which become 

 different in type from the parent cell. Neither do the endothelial 

 cells lining the vessel walls change into chromatophores or into 

 erythroblasts, or vice versa. 



From all the observations on these yolk-sacs we must conclude 

 that the four types of cells described above have developed from 

 four different anlagen, although these anlagen were not neces- 

 sarily localized groups of cells, but were diffusely scattered 

 mesenchymal cells capable of developing into a definite product, 

 either normal or abnormal, depending upon the nature of the 



