ORIGIN OF BLOOD AND ENDOTHELIUM ' 295 



may result merely from the fact of the intimate connection of 

 the sclerotom and intermediate cell mass before their original 

 separation. 



The question now arises whether all the blood of the Teleost 

 embryo is exclusively derived from the intermediate 'Blutstrange.' 

 Felix admits that the endothelium of the glomerular vessels of 

 the mesonephros arise in loco and at the same time blood cor- 

 puscles often occur in this region. Sobotta claims that in the 

 vascular network in the tail of the trout embryo some of the 

 blood corpuscle anlage exists. 



Both of these exotic positions of origin may be easily under- 

 stood. In the first place, the nephric anlage is formed from cells 

 in direct association with those constituting the early intermediate 

 cell mass, and in the separation it probably happens that some 

 future blood cells are held within the kidney anlage and these 

 cells later develop in their proper fashion. The presence of 

 blood corpuscles in the vascular network of the tail is due to the 

 fact that the intermediate cell mass in many Teleosts, as Marcus 

 ('05) has pointed out and as Senior ('09) particularly emphasized 

 extends far back into the caudal region. 



A similar consideration is the question of origin of vessels 

 from material other than that of the intermediate cell mass 

 and sclerotom. This is also important, and numerous observa- 

 tions would indicate that in the early bony fish embryo vessels 

 unquestionably arise in loco and not solely as outgrowths or 

 sprouts from a central vessel anlage. Sobotta ('02) on the con- 

 trary imagines a gradual growing away of the vascular system 

 from its local origin, the sclerotom. The aorta is the primary 

 vessel and, for example, the sub-intestinal vein arises from the 

 aorta by vascular sprouts which grow around the gut, broaden 

 out and fuse on its ventral side and finally give rise to the longi- 

 tudinal vein. This theory of Sobotta is as unacceptable in the 

 face of the great body of evidence to the contrary, Felix ('97), 

 Riickert ('88), Hahn ('09) and many others, as is the opposite 

 ingrowth parablast theory of His ('75). < 



The consideration has been confined so far to the intra-embry- 

 onic blood vessels. We may now briefly discuss the develop- 



