124 FRANKLIN P. REAGAN 



So far as endothelium is concerned, the two cases are most 

 certainly not parallel. There is abundant evidence for the be- 

 lief that endothelium can form locally in the mesenchyme in 

 practically all parts of the germ layer of which it is a constitu- 

 ent. There is no evidence that pancreatic cells form locally in, 

 or even migrate into practically all regions of the entodermal tis- 

 sue. One can predict, in case of pancreatic and liver tissues, 

 approximately the very region in which the anlagen will make 

 themselves manifest; to say the very least, one can predict that 

 in the greater part of the entoderm no pancreatic or liver anlagen 

 ever exist. This can not be said to be true of mesenchyme cells 

 capable of forming endothelium. If the results outlined in my 

 own work hold good for teleost blood-cells, the development of the 

 latter will likewise be found not to constitute a case comparable 

 to that of the development of pancreas and liver. 



The inabiUty of one differentiated tissue to replace or regen- 

 erate another has absolutely no bearing on the question. In 

 some species a given organ can be regenerated while in other 

 closely related species that very same organ can not. Obviously 

 a comparison of the regenerative power of different tissues of the 

 same organism or of their ability to replace each other has no im- 

 portance for the question at hand. The ordinary process is not 

 for a destroyed tissue to be replaced by another equally differen- 

 tiated tissue, but to be regenerated by a tissue intermediate be- 

 tween the two and more primitive than either. 



The monophyletic view does not hold that development shall 

 be chaotic and without system. For the monophyletic view it 

 is not necessary that the diverse mesenchymal derivatives should 

 invariably be found in close association. If it be true that there 

 is really an equipotential stage in which the fate of a mesen- 

 chyme cell is determined in part by factors extraneous to its 

 own constitution, it does not necessarily follow that the factors 

 which direct the development of these cells into different avenues 

 will all operate at the same place or at the same time in that 

 truly equipotential system. Neither is the possibility pre- 

 cluded that in some parts this equipotentiality or embryonic 

 condition may persist until a late stage in the ontogeny, making 

 possible a plasticity of interadaptation and a regulation of parts 



