88 FRANKLIN P. REAGAN 



Whatever may be the significance of these conditions above 

 described, they demonstrate the fact that projections from the 

 entoderm resenibHng pre- vascular tissue undoubtedly can be 

 found. In view of their demonstration we have these alterna- 

 tives of interpretation: either the entoderm is capable of giving 

 rise to vascular tissue, or such proliferations and projections from 

 any tissue whatsoever are to be distrusted as indications of the 

 actual origin of the vascular tissue. If one searches with suf- 

 ficient diligence, he is likely to become convinced that neither 

 the view of the entodermal origin, nor the view of the mesoder- 

 mal origin of the vascular tissue embodies the whole truth; con- 

 versely, he is likely to be convinced that each view contains an 

 element of truth. The two views are best harmonized by con- 

 sidering the pre- vascular tissue as mesenchyme, whatever may 

 be its source. If cardiac endothelium actually arises from ento- 

 derm, the phenomenon may well be considered a mere local 

 adaptation to the circumstance that it is advantageous to have 

 the bilateral endocardial tubes closely approximated in antici- 

 pation of their future fusion. The whole question may well re- 

 main open, whether one or both layers are capable of giving rise 

 to vascular tissue. It seems highly probably that both layers 

 share in the production of endothelium. Subsequent observa- 

 tion alone can determine the significance which is to be attached 

 to these undoubted conditions which I have actually observed, 

 and have figured as faithfully as possible. 



3. The origin of endothelium in hybrid teleosts 



Loeb (28) first demonstrated that chemical treatment or hy- 

 bridization in the teleost would inhibit the development of. a 

 blood-circulation, even though the embryo was able to develop a 

 beating heart. Stockard (64) has shown and also Werber (78) 

 has shown in case of chemical treatment, that the lack of circu- 

 lation is caused by the failure of the independent vascular anla- 

 gen to become continuous. McClure (37) has extended these 

 observations to the lymphatics. The effect of hybridization 

 was again observed by Newman (45). Thorington and the writer 



