ORIGIN OF VASCULAR TISSUES 123 



of a predestination. If, on the other hand, otic capsules nor- 

 mally were in the habit of developing in practically all parts of 

 the embryonic mesenchyme, such an experiment would have 

 little significance; in that case, all that could be said is that there 

 would be no especial reason for the belief that the formation of 

 the otic capsule was a matter of predestination; preformation in 

 this instance could neither be proved nor disproved. Suppose 

 for instance that some substance were discovered which would 

 cause the formation of endothelium wherever that substance 

 might be introduced into the mesenchyme. If it be admitted 

 that there are mesenchyme cells in practically all regions which 

 are capable of forming endothelium, it would obviously be im- 

 possible to say whether the endothelium which had resulted from 

 the introduction of the substance came from mesenchyme cells 

 whose fate had previously been sealed, or from truly indifferent 

 cells whose vascular characteristics had actually been initiated 

 and not merely unfolded, awakened, or stimulated. 



By common consent we have generally agreed that there is no 

 especial reason for believing that a widely diffused mesenchymal 

 derivative is specific. It is generally agreed that before a tis- 

 sue can be claimed to be specific it must conform to three con- 

 ditions: First, it must have a demonstrable and narrowly lim- 

 ited anlage whose position we can predict. Second, this tissue 

 must never be found producing any kind of tissue different from 

 the one for which the anlage is claimed to be specific. Third, 

 the anlage producing this tissue must never produce any other 

 kind of tissue. If a mesenchymal derivative fails to conform to 

 any of these conditions, there is no especial reason for believing 

 it to be a 'specific' tissue. 



In his final conclusion (65, p. 323) Stockard states : 



The facts here presented seem to indicate that vascular endothelium, 

 erythrocytes, and leucocytes, although all arise from mesenchjone, are 

 really polyphyletic in origin : that is, each has a different mesenchymal 

 anlage. To make the meaning absolutely clear, I consider both the 

 origin of the liver and pancreas cells a parallel case; both arise from 

 entoderm but each is formed from a distinctly different entodermal 

 anlage, and if one of these anlagen is destroyed the other is powerless 

 to replace it. 



