150 
ZOOLOGY: H. E. JORDAN 
had already reported similar structures in the chick. Aortic cell 
clusters would seem to be a common feature of certain early stages 
of vertebrate development. 
All of the above-named investigators agree in interpreting the con- 
stituent cells of the clusters as hemoblasts, variously designated as 
^lymphocytes/ ^mesameboids/ progenitors of macrophages ? hemo- 
blasts. Minot^ alone disagrees with the otherwise unanimous con- 
clusion that they represent endothelial differentiation products. That 
they are endothehal derivatives, however, rather than accretion 
products of hemoblasts from the circulating embryonic blood is easy of 
demonstration. Aortic cell clusters represent one phase of the general 
hemogenic capacity of embryonic endothelium. 
The doctrine of the partial endothehal origin of hemoblasts from 
embryonic endothelium has become associated with the monophyletic 
hypothesis of blood cell origin (Maximow)i that of the non-hemogenic 
capacity of endothelium with the polyphyletic and diphyletic hypotheses 
(Stockard).^ The question of the genetic relationship between endo- 
thelium and certain cellular elements of the embryonic blood touches 
also the 'angioblast' theory of His. The two chief tenets of this theory 
are : (1) the inability of intraembryonic mesenchyme to produce blood 
vascular tissue, and (2) the incapacity of endothelium to differentiate 
into blood cells. Abandonment of the first tenet has been forced 
largely through the experimental work of Hahn,^ of Miller and 
McWhorter,^ of Reagan, and of Stockard;^ and the morphologic 
studies of Schulte^^ on the cat embryo, those of McClure^^ on the 
trout embryo, and the studies of Huntington^^ on the development 
of the lymphatics in amnio tes. The surrender of this portion of the 
original theory is gradually being made by some of its staunchest advo- 
cates (vide Sabin^^) . The disproof of the second tenet is the chief burden 
of this investigation. 
The material studied includes three mongoose embryos of 5, 6, and 
7 mm. respectively, a series of pig embryos of from 8 to 12 mm., chick 
embryos of the third and fourth day of incubation, and a series of twenty 
loggerhead turtle embryos ranging from the second to the thirty-second 
day of incubation. 1^ These embryos are variously preserved and stained, 
the several methods including fixation with Helly's fluid and staining 
with Giemsa's solution. 
This description confines itself almost exclusively to the 5 mm. mon- 
goose embryo and the 12 day turtle embryo. This selected material 
is at just the proper stage of development to furnish the key for the cor- 
rect interpretation of the larger aortic clusters of the 10 mm. pig embryos. 
