ORIGIN OF BLOOD AND ENDOTHELIUM 301 



part of the somites that was immediately in contact with the inter- 

 mediate cell mass portion of the primary seitenplatte. As the forward 

 somites bud off sclerotoms, these also are added to the 'mesenchy- 

 maortenstrang.' 



The median part of the 'strang' forms the aorta, 'aortenstrang/ 

 the lateral the 'mesenchymgewebe' (mesenchymestrang) . The 'aorten- 

 strang' is at first solid and does not obtain a continuous lumen to begin with, 

 but here and there develops a space, and these spaces become confluent to 

 form the tubes and build the paired aortae. Certain portions of the 

 Strang remain solid much longer than others. The association of the 

 paired aortae to form an unpaired single vessel soon follows. While 

 the aorta is being so formed, one never finds blood cells within its lumen. 

 Blood cells occur only in the 'venenstrang' and in certain vessels of the 

 nephric glomeruli. Occasionally certain of the glomerular vessels 

 contain blood corpuscles at a time when the blood circulation is not 

 yet closed. 



Felix ('97) cites the observation of P. Mayer ('94) on very 

 young Selachian embryos in which the medulary tube was still 

 open. It was found in such embryos that the aorta is segmental 

 and derived from the somites and subsequently the longitudinal 

 tube is formed by the fusion of these isolated points. Felix 

 agrees with P. Meyer's observations from his study on the 

 Teleost. 



There has been great diversity of opinion regarding the germ 

 layer from which the vascular endothelium and blood corpuscles 

 arise. In the Hterature it may be found that certain competent 

 investigators have in each vertebrate class claimed the vascular 

 endothelium and blood cells to be derived from the endoderm, 

 while other workers of equal authority have found the vessels 

 and corpuscles to arise from the mesoderm. The consistency 

 of the disagreement which one finds in a review of this literature 

 is most peculiar. These disagreements have their foundation 

 in the extreme difficulty of the problem on fixed material. 



It is interesting to note that in no case has the same author 

 derived the blood and vessel endothelium from different germ 

 layers. Each author always takes the position that blood and 

 vascular endothelium arise from either the mesoderm or the 

 endoderm. 



We have here much to do with wandering cells which become 

 lost from their epithelial layer, and it is impossible to state their 



