120 FRANKLIN P. REAGAN 



a thesis. It has not yet occurred to anyone to claim that the en- 

 dotheUal cells forming in practically all regions of the body have 

 migrated there from other sources or from any single source. 

 If, however, it should be shown that such migration of erythro- 

 cyte-forming mesenchyme within the body is a process of im- 

 portance in the normal development which Stockard's experi- 

 mental conditions have inhibited, the demonstration of this fact 

 is of importance. Our ultimate aim is an understanding of the 

 normal process. If unfortunately it be true that chemical treat- 

 ment is not a panacea for our perplexities concerning the normal 

 processes of vascular development, it is desirable that we realize 

 that fact. 



Let us now consider certain of the deductions made by Stock- 

 ard from his results which are said to render the monophyletic 

 view extremely improbable. If my analysis be correct, his en- 

 tire philosophy of vascular development rests on the following 

 assertion, in favor of which his own work offers no proof. On 

 page 229 (65) he makes this statement which serves as his point 

 of departure: ''There can be no doubt of the greiit genetic dif- 

 ference between blood-cells and connective tissue cells, yet their 

 parent cells are with our present methods indistinguishable." 

 As a matter of fact the doubt which has been entertained and 

 which still exists as to the great genetic difference of blood and 

 connective tissue is the basis of the very problem with which we 

 are concerned. In the next sentence following the one just 

 quoted it is stated: "We may with equal justice go further and 

 hold likewise that the cells from which vascular endothelium, 

 red blood cells and white blood cells arise are mesenchymal cells 

 really differing in nature according to whether they will give rise 

 to one or the other of the three types." This statement is de- 

 pendent on the one preceding in which we are asked to accept 

 from authority that preformation follows as a matter of course 

 since the end-results are different. On pages 315-316 (65) we 

 have this same idea carried out still farther. Stockard states : 



This invisible difference determines the destiny of the cell to form 

 either leucocytes or erythrocytes, but we can not stop just at this 

 point; we must go back to the actual beginning: Then it is found that 



