460 H. E. JORDAN 
young turtle embryo I have seen an eosinophilic granulocyte 
within an early blood-island, otherwise differentiating largely 
into erythrocytes. Until it-can be actually demonstrated that 
the environment is identical in such instances, these facts do not 
contradict, but sustain the monophyletic view. 
Unless we assume an identity of developmental potencies in 
the case of the hemoblastic mesenchymal cells, one is forced to 
the position that the mesenchymal ancestors of fat cells, pig- 
ment cells, endothelium and smooth muscle cells also differ in 
respect of specific developmental potencies. But such unquali- 
fied position must ignore a large group of histogenetic data. 
Mesenchyme cells are apparently of identical nature and en- 
dowed with multiple potencies, that is, ‘equipotential’ and 
‘polyvalent.’ The expression of any one type of mesenchymal 
differentiation—whether as endothelium, erythrocytes, granu- 
locytes, or fat—seems accordingly dependent upon extrinsic 
factors of a type or degree not yet detectable or measurable by 
our means of investigation. Indeed, Danchakoff describes endo- 
thelial cells of the spleen graft separating from the wall of small 
blood-vessels both centrally, where they pass into the lumen as 
hemoblasts and differentiate into erythrocytes, and peripherally 
where they become reincorporated with the mesenchyma and 
may redifferentiate into granuloblasts. I haye observed com- 
parable phenomena in the bone marrow and the body mesen- 
chyma of the turtle embryo. These observations show that the 
same cell, an endothelial hemoblast, may become either an 
erythrocyte or a granulocyte according as the environmental 
stimuli change ever so slightly. They furnish, moreover, the 
very strongest support to the monophyletic theory. But they 
show also that adjacent identically differentiated cells, namely, 
young endothelial cells, suffer divergent further differentiation 
in an apparently identical environment, namely, inclusion in an 
endothelial wall with luminal and mesenchymal surfaces. But 
since the cells are identical, i.e., mesenchymal cells slightly 
differentiated into functional young endothelal cells, then the 
environment of extrinsic stimuli determining the direction of 
migration and the initial steps in the further differentiation into 
