ORIGIN OF BLOOD AND ENDOTHELIUM 289 



Finally, one may conclude that the blood cells like many other 

 specific tissues and organs have a definite localized specific 

 anlage and that this anlage is distinct and separate in most 

 cases from that of the vessel linings. In some cases, however, 

 the blood and endothelial anlagenmay come into intimate associa- 

 tion, yet even here the two are probably of different mesenchymal 

 origins. 



A CONSIDERATION OF THE EXPERIMENTAL STUDY ON THE ORIGIN 

 OF BLOOD IN TELEOSTS IN RELATION TO THE MORE 

 RECENT STUDIES ON THE ORIGIN AND DEVELOP- 

 MENT OF VESSELS AND BLOOD CELLS 



1. Introduction 



This experimental study of the origin and development of 

 blood and vessels relates itself to three more or less separate 

 fields of investigation. 



In the first place, the manner in which the blood anlage in 

 Teleosts has separated itself as a unique intermediate cell mass 

 has caused it to be studied as a special subject somewhat isolated 

 from the more general literature on the development of blood 

 in other vertebrates. Yet one very soon appreciates the mis- 

 take of this isolation since contributions such as those of Felix 

 ('97) and Swaen and Brachet ('99, '01, '04), in particular, on 

 the Teleosts are of more general importance than most investi- 

 gations dealing with the broad subject of blood development 

 in the vertebrates. The very fact that in this group the blood 

 anlage is so pecuharly localized in the embryo lends itself as a 

 great aid to the solutions of many questions of haematopoesis 

 or blood genesis. 



Secondly, a consideration of the origin and formation of the 

 heart lining or endocardium and the vascular endothelium, in 

 these embryos which have developed without having had plasma 

 or fluids to circulate within their vessels, may furnish much 

 important data towards a final solution of the origin and signifi- 

 cance of endothelial lining cells, and the manner of spread and 

 distribution of such cells through the embryonic body and the 

 yolk-sac. 



