ORIGIN OF BLOOD AND ENDOTHELIUM 95 



A point of much importance is the fact that neither these hearts 

 with their endothelial linings nor any portion of the aortae at any 

 stage of development have ever been seen to contain an erythroblast 

 or an erythrocyte. Cells of this type are completely absent 

 from the anterior region of the embryo. 



4. Pigment cells normally occur on the Fundulus yolk-sac 

 and arrange themselves along the vascular net so as to map 

 out the yolk-sac circulation in a striking manner. Loeb has 

 thought that this arrangement along the vessel walls was due 

 to the presence of oxygen carried by the corpuscles within the 

 vessels. In the embryos without a yolk-sac circulation the 

 pigment cells arise but rarely become fully expanded so that 

 the usual long branched processes are represented only by short 

 projections, the chromatophore consequently seems much smaller 

 than usual. 



The unexpanded pigment cells, however, wander over the 

 yolk-sac and collect in numbers around the plasma filled spaces. 

 The yolk surface of the pericardium and the periphery of the 

 Kupffer's vesicle are often almost covered with pigment. The 

 hearts are during early stages full of plasma and the pigment 

 cells form a sheath around them, while pigment cells are never 

 present on the normal hearts during the embryonic period. 



These facts would seem to indicate that the plasma rather 

 than the erythrocytes contain the substance which attracts 

 the chromatophores and initiates then* arrangement along the 

 normal vascular net of the yolk-sac. 



5. A definite mass of cells characteristic of the Teleost embryo 

 is located in the posterior half of the body between the notochord 

 and the gut and extends well into the tail region. This so- 

 called 'intermediate cell mass' is the intra-embryonic red blood 

 cell anlage in many of the species. 



The peripheral cells of the mass as claimed by Swaen and 

 Brachet or the mesenchyme about the mass, Sobotta, forms a 

 vascular endothelium which encloses the central early erythro- 

 blasts. 



In individuals without a circulation the erythroblasts arise 

 in a normal manner in this centrally located position, and be- 



