ORIGIN OF BLOOD AND ENDOTHELIUM 299 



and Brachet, would derive the endocardium and aorta also from 

 this common source. 



The important fact is that in the small pelagic embryos, 

 where no blood formation takes place before hatching, the inter- 

 mediate cell mass forms the aorta and the cardinal veins and is 

 also derived from the lateral plate. The lateral plate thus con- 

 tains cells capable of forming vascular endothelium, and this is 

 the case in all vertebrates. 



At an early time in evolution the extra-embryonic blood form- 

 ing mesoderm has been included within the body of the Teleost 

 embryo and lies over the gut as the intermediate cell mass repre- 

 senting the yolk vascular layer. Here it is important to note 

 that the yolk-sac of the Teleosts contains no real mesodermal 

 layer, only separate wandering mesenchymal cells are found be- 

 tween the ectoderm and periblast, and these wandering cells 

 have migrated out from the embryo. 



Finally, as MoUier ('06) states in his review of this subject, 

 it is not a question of the formation of the intermediate cell 

 mass in the individual bony fish, but the wider question of the 

 behavior of the blood forming peripheral mesoderm in the bony 

 fish. All of the results must be considered in this light in their 

 application to other animal classes. 



The intra-embryonic blood formation in the bony fish does 

 not represent the primitive type for vertebrates as Sobotta ('02) 

 claims, but this is, no doubt, a modified secondary condition 

 accompanying the various other modified and special develop- 

 mental processes which bony fish embryo so frequently presents. 



Wilson ('91) states of the mesoderm of the Teleost that: ''The 

 ventral subvitelline mesoderm, having in this way losts it func- 

 tion in the Teleost, must be regarded as a rudimentary organ 

 of the gastrula. It always remains very small, and does not 

 form any special organ or set of organs in the embryo." The 

 real fact is that the subvitelline mesoderm is misplaced, being 

 within the embryo as the intermediate cell mass and here forms 

 the blood of the individual and, therefore, the yolk-sac of the 

 bony fish has no mesodermic layer. 



