ORIGIN OF VASCULAR TISSUES 117 



pared with many other blood-producing tissues. The relatively 

 few cases of this sort are generally regarded as significant prin- 

 cipallj' for the evidence which they afford of the non-specificity 

 of endothelium. 



It is conceivable that a tissue whose hematopoetic tendencies 

 are weak would be the first to be affected by abnormal treat- 

 ment. Stockard reasons somewhat differently. He states that 

 "if vascular endothelium had such power, then one might expect 

 that this power would show itself where it is most needed, for 

 example in these embryos in which the blood never circulates." 

 One might take the equally teleological, but opposite view that 

 these embryos are the very ones which do not need erythrocytes. 

 Stockard goes still farther and maintains (65, p. 311) that the al- 

 coholic environment should be able to stimulate endothelium to 

 form blood cells, provided endothelium is capable of being so 

 stimulated. He points out that no endothelial cell has ever 

 been seen to give rise to a blood cell. Should this observation 

 ever be made, it becomes of interest to know whether the phe- 

 nomenon would be regarded as adequate proof of the ability of 

 endothelium to form blood, or whether it might be regarded as 

 an instance in which the blood anlage was 'exotic' (65, p. 395) 

 or one in which the process of development had not been clean- 

 cut (65, p. 292). 



Concerning the following points it is difficult to determine 

 definitely the interpretation to be placed on Stockard's state- 

 ments. He figures leucocytes in very young stages, yet (65, p. 

 309) regards their late appearance as significant. He states (65, 

 p. 280) that they have never been observed except in the anterior 

 region of young embryos, yet believes (65, p. 284) that they are 

 of a wandering disposition, later making their way into all parts 

 of the embryo. Rather strangely, on page 269 (65) he states 

 that ''in no case has any type of lymphocyte or leucocyte been 

 present in these yolk-islands except as late wandering cells," 

 whereas he repeatedly states that yolk-islands are always devoid 

 of such structures in the absence of circulation. He regards en- 

 dothelial cells equally specific with blood, and far removed geneti- 

 cally from connective tissue cells and blood cells; he classes endo- 



