560 
ZOOLOGY: C. R. STOCKARD 
8. Vascular endothelium arises in loco in many parts of the embryonic 
body other than those localities in which blood cells form. The endo- 
thelium is in all cases utterly incapable of giving rise to any type of 
blood cell. This incapacity cannot be attributed to the abnormal con- 
dition of the embryo, since true blood cell anlagen in the same speci- 
men produce blood corpuscles in abundance. 
The lining of the vessel walls in the fish embryo has no blood form- 
ing function. 
9. Neither lymphocytes nor other types of white blood corpuscles, 
have been found to arise in the yolk-sac blood islands, nor within the 
intermediate cell mass. The embryonic white blood cells are found 
most abundantly in the anterior body and head regions, and there occupy 
extra-vascular positions usually lying among the mesenchymal cells. 
The sources of origin of the white and red blood corpuscles in Fundulus 
embryos are distinct, and these two diferent types of cells cannot be con- 
sidered to have a monophyletic origin except in so far as both arise from 
mesenchymal cells. 
The adult blood of Fundulus contains lymphocytes and several varie- 
ties of granular leucocytes. 
10. There is evidence to indicate that definite environmental condi- 
tions are necessary for blood cell proliferation or multiplication. Blood 
cells do not normally divide when completely enclosed by vascular endo- 
thelium. This is the key to the shifting series of so-called haematopoetic 
organs found during embryonic development. 
Erythroblasts, embryonic red blood corpuscles, lying about spaces 
unenclosed by vascular endothelium, proliferate steadily and give off 
their products into the space from which they find their way into the 
embryonic vessels. Should such an erythroblast be carried by the circu- 
lation to another unlined space, it may become arrested there and again 
undergo a series of divisions, giving rise to other erythroblasts. When, 
however, these spaces become lined by endothelium, the blood cell 
reproduction stops. 
In most embryos the earliest blood cell formation occurs in the yolk- 
sac blood islands. The cells in these islands continue to divide until 
they become surrounded by endothelium, then the yolk-sac blood islands 
lose their haematopoetic function and become a vascular net through 
which the blood circulates. The liver now takes up the role of har- 
boring dividing blood cells within its tissue spaces. When these spaces 
become vascularized by endothelium, here again the blood cells no 
longer multiply but merely circulate. 
Finally, in the mammalian embryo, one organ after another ceases 
