84 FRANKLIN P. REAGAN 



iiig ill the iioniuxl individual to have arisen from entoderm. A 

 most striking case of this sort was reported by Lee (27). It is of 

 importance to know whether inthnacy of endothehum and ento- 

 derm is of primary or of secondary significance. It is highly 

 probable that the association of the two tissues is in some way 

 causally corinected with the fusion of the bilateral endocardial 

 tubes. This might be brought about in one of, or a combina- 

 tion of two ways. First, endothelium of mesodermal origin 

 might approach and become applied to the entoderm at a very 

 early stage in its genesis; second, this intimacy of connection 

 might be a genetic one; it might be that the assumption of the 

 role of vascular proliferation is merely a local adaptation for 

 the process of heart-formation in the amniotes. Either of these 

 two processes might better insure the meeting of the endothelial 

 tubes, following the withdrawal of the intervening entoderm. 

 The immediate section of the discussion is devoted to the second 

 possibility. 



I shall now present what I believe to be actual instances of 

 entodermal proliferation of prevascular mesoderm. It may be 

 that I have attached entirely too much importance to these ap- 

 parent cases of endothelial proliferation by entoderm. If this 

 be true, it has been for the purpose of putting to as severe a test 

 as possible the view in favor of which I have at times been preju- 

 diced — namely the view which regards the mesoderm as the 

 only possible source of the prevascular tissue. 



In the posterior region where endothelium is unquestionably 

 of mesodermal origin, it was seen that cells were moved bodily 

 from the more or less ragged surface of the mesoderm. When 

 such cells could first be recognized as forerunners of the vascu-' 

 lar tissue they were invariably more closely associated with meso- 

 derm than with entoderm. In the middle heart-region this was 

 often observed not to be the case; the vascular cells seemed from 

 the first to be more closely related to the entoderm than to the 

 mesoderm. At the anterior and posterior limits of the heart, 

 transitions between these extreme conditions exist. Now it is 

 not characteristic of entodermal cells to leave their epithelium 

 and wander about as such in the interstitial regions. If cardiac 



