ORIGIN OF VASCULAR TISSUES 121 



although two wandering mesenchyme cells on the yolk of the fish 

 embiyo are indistinguishable so far as observations go, yet they are 

 fundamentally different since one is destined to form an erythroblast 

 wliile the other possesses no such power and can only form an endo- 

 thelial lining cell or pigment cell as the case may be. This is all the 



diph3detic or polyphyletic school would ask Here we 



logically stop, for this is what is conceived by embryologists to be an 



anlage To stop with the tissue anlage we find strong 



evidence that certain mesenchymal cells are designated to form erythro- 

 blasts, others leucoblasts, and still others, and these are more uni- 

 versally scattered throughout the body, give rise to vascular endo- 

 thelium. 



On page 283 (65) Stockard states that the "mesenchymal cell, 

 if taken early enough could no doubt give rise to other mesen- 

 chymal cells which would later form these types of cells." 



Thus it would seem that our 'conceived' anlage is not a very 

 tangible thing. We could not definitely locate such an anlage 

 and be sure that it might not be a mesenchyme cell which might 

 give rise to other mesenchyme cells 'of these various types.' 

 Its existence is assured by authority and not by demonstration. 

 Some place in the morphologically indifferent stage which we 

 can not locate, 'we find strong evidence' that certain mesenchyme 

 cells are designated to this or that fate. But if it be true that 

 'here we logically stop' (i.e., looking retrospectively), the con- 

 verse must be true that looking prospectiifely, 'here we logi- 

 cally start.' We have selected an instant in which the cell-com- 

 plex appears by all known means of observation to be indiffer- 

 ent, but our Intuition tells us that differences exist. Prior to 

 this time none of the diverse progeny could be said to have an 

 anlage. Now if it be the goal of the polyphyletic school to prove 

 that with actual differences once given, the remainder of the proc- 

 ess is, in general, one of greater divergence, their goal is easily 

 reached. Doubtless no one would oppose them. True enough 

 the word 'polyphyletic' etymologically conveys the idea of 

 'many branches.' If this idea has merely to do with the fate of 

 the parts subsequent to the instant at which we 'logically start,' 

 — if in other words our phyleticism has nothing to do with 

 the origin of the parts beyond the point at which we 'logically 

 stopped,' then the general process of development is uncondi- 



