ORIGIN OF VASCULAR TISSUES 115 



ir. GENERAL CONCLUSIONS 

 1. Does abnormal treatment obscure the normal process? 



At this point I wish to consider certain phases of Stockard's 

 work, taking up first his results, and then the conclusions which 

 he drew from them. In commenting on his previous work (65) 

 he stated (66, p. 583) that this work seemed "in the light of 

 past literature to render highly probable, if not to prove, the 

 polyphyletic origin of the various types of blood-cells," thus dis- 

 posing of the ''now extremely improbable monophyletic view." 

 He regards his technique of study as so highly satisfactoiy that 

 ''the only disadvantage is that the worker may be led to wonder 

 whether so apparently a simple problem is of scientific impor- 

 tance." He believes himself to have disproved the monophy- 

 letic view by demonstrating that eiythrocytes of the teleost 

 arise only in the intermediate cell-mass and on the posterior 

 yolk; that leucocytes originate only in the anterior end of the 

 body; that erythrocytes cannot develop from endothelium, from 

 anterior mesenchyme, in anterior vessels, on the anterior and 

 ventral yolk and in the liver. 



The first question which every one must decide for himself is, 

 whether he will accept the introductory statement of Stockard 

 (65, p. 234) that "it can not be argued, so far as the blood anlage 

 is concerned, that the conditions recorded are pathological or 

 other than those which would occur in the normal genesis of the 

 blood except that it never circulates." If it be true that such 

 argument is impossible, then the whole question is settled once 

 for all. There would seem to be a possibility that it can be ar- 

 gued, and perhaps even proved that the blood anlage has shared 

 in the afflictions which are evident in all the other tissues in 

 Stockard's figures. What would exempt the blood anlagen from 

 the ill-effects demonstrable in the other tissues? These are 

 questions which one should answer before proceeding to other 

 considerations. 



From Stockard's own experiments and the experiments of 

 many other observers, it is evident that the anterior end of the 



