ORIGIN OF BLOOD AND ENDOTHELIUM 91 



group. The position determines the direction of their develop- 

 ment and only in certain places does one find all types being 

 formed, for example, in the embryonic liver and the adult bone 

 marrow. 



In reply to the extreme monophyletic position it may be asked: 

 Why are only erythrocytes present in the old blood islands on 

 the yolk of non-circulating specimens? Why is no cellular 

 blood element present in the aorta and other endothelial lined 

 vessels in the anterior region of similar embryos? Why are 

 wandering "primitive blood cells" unable to form blood in the 

 liver and other positions while blood forming power is present 

 to a vigorous extent in certain regions of the same embryo but 

 from a definite anlage? Wandering cells in the Teleost embryos 

 have to do with yolk-sac blood origin, but these wandering cells 

 are the equivalent of a part of the peripheral mesoderm and 

 always wander out on to comparable regions of the yolk and 

 never wander to other places within the embryo. 



Maximow probably criticizes correctly the many artificial dis- 

 tinctions between various leucocytes which are pointed out by 

 some advocates of the polyphyletic theory. My experiments 

 do not bear on this point up to the present time. 



Finally Maximow does not imply that a granular leucocyte 

 or red corpuscle can change into anything else, they cannot un- 

 differentiate. He states that before there is any development 

 of granulation or haemoglobin in two different cells, there is 

 really a difference in the cells though we are unable to distinguish 

 it. This invisible difference determines the destiny of the 

 cell to form either a leucocyte or erythrocyte. This is certainly 

 true, but we cannot stop just at this point; these differences must 

 be carried a step further or really back to their actual beginning. 

 Then it is found that although two wandering mesenchyme cells 

 on the yolk-sac of the fish embryo are indistinguishable so far 

 as our powers of observation go, yet they are fundamentally 

 different since one is destined to form only an erythroblast 

 while the other possesses no such power and can form only an 

 endothelial lining cell or pigment cell as the case may be. 



