VASCULOGENESIS IN THE CAT 27 



This granted, it is arbitrary to assert, that injection once it 

 becomes practicable, dispenses us from the examination of the 

 periphery beyond the terminus of the injection, where it is pos- 

 sible, where there is even antecedent probability that processes 

 analagous to those in the 'anlage' continue and play an impor- 

 tant role in the increase of vessels within the embryo. Whether 

 this is true or not must be decided by the methods which do not 

 prejudge the question. As in the case of the 'anlage' the results 

 of the study of sections are in agreement with intra vitarn find- 

 ings, it is but reasonable in later stages and in the body of the 

 embryo, where intra vitam methods are as yet inapplicable, to 

 credit the method of sections and reconstruction with the same 

 validity it is proved to possess in early stages. 



To deny this, and insist that the limited field explored by in- 

 jections is alone accessible to investigation, is not to assert the 

 prunacy of the injection method, but to fall back upon an ancient 

 logical de\'ice known as the petitio principii. 



The final object'on and absolute disproof of the doctrine of 

 specificity is the heterogeneity of the products to which endo- 

 thelium gives rise." I have already mentioned Huntington's de- 

 scription of the conversion to mesenchyme of regressive vessels 

 in embryos of the cat {vide ante, p. 12). 



Aurel von Szily^" has described and illustrated the production 

 of mesenchjane from the endothelial tube of the heart. And to 

 pass over other investigators who have glanced the subject, 

 Mall" has recently followed the process in detail and noted its 

 theoretical importance. 



The great importance of this distinction is at once apparent for it 

 shows that connective tissue arises also from endothelial cells and that 

 the intima of the entire vascular sytem includhig the valves of the heart 

 has a like origin. 



Further study will probal:)ly sliow that endotlielial connective tis- 

 sues are l)y no means of rare occurrence. 



These observations of v. Szily, Huntington and Mall are of 

 decisive importance and seem finally to dispose of the doctrine 



" The idea that endothelium gives rise to connei'tive tissue was clearly ex- 

 pressed as early as 1872 by Franz Boll. Arch. f. Mikro. Anat., Bd. 8, p. 53. 

 « 1903, Anat. Anz., Bd. 24, p. 417, cf. especially figure 2. . 

 " 1912, Aiiipr. Jour. Anat. vol. 13, p. 249. 



