ORIGIN OF BLOOD AND ENDOTHELIUM 261 



The hearts in these experimental embryos as a rule lead directly 

 into a more or less well formed aorta which, in all cases shows 

 an endothelial lining. The arches arising from this ventral aorta 

 are very variable in the different embryos, yet in some cases 

 are formed in an almost normal fashion. These arches also 

 show a beautifully formed endothelial lining, but here again one 

 is impressed by the absolute absence of erythroblasts in any 

 stage of development within the neighborhood of the heart 

 or aortic endothelia. 



2. The "intermediate cell massf^ its origin, position and significance 

 as an intra-embryonic blood anlage 



On tracing the sections posteriorly one finds the intermediate 

 cell mass to begin caudad of the pectoral fins and in the region 

 of the anterior portion of the kidney duct. In studying a pro- 

 gressive series of very young stages forty-eight, sixty-six and 

 seventy-two hours, the intermediate cell mass may readily be 

 demonstrated to arise from the lateral mesodermic plates in the 

 manner so clearly described by Swaen and Brachet ('01, '04), 

 Felix ('97) previously pointed out that the primary lateral 

 mesodermic plates extend away from the somite and later be- 

 come divided into the following three parts. The median cells 

 lying close to the somites separate away to form a continuous 

 longitudinal string, the string from each side forming one lateral 

 half of the future intermediate cell mass. The intermediate 

 cells of the primary lateral plates just lateral to the above 

 median group give rise to a second cord of cells which later 

 forms the primary nephric duct. The remaining lateral layer 

 of cells now constitutes the secondary lateral plates which 

 split to form two lamellae. 



The primary lateral plate mesoderm thus gives rise to the 

 intermediate cell mass, the primary nephric ducts and the 

 somatic and splanchnic mesodermic layers of the lateral body 

 wall. Between these two lateral mesodermic layers arises the 

 portion of the coelomic cavity which we have seen in the living 

 embryos without a circulation of the blood to be greatly dis- 

 tended with fluid. 



