BLOOD AND BONE-MARROW OF FROG 461 
different varieties of blood corpuscles, must indeed have differed 
to some extent, however slightly. 
An interesting feature of this marrow concerns further the 
fact of a more or less sharp grouping of the several types of cells 
at the various phases of development, very much as is the case 
in the red marrow of the femur of the pigeon and of the rabbit 
during stages of intense hemopoiesis, that is, a certain group of 
cells comprises predominantly small lymphocytes, another 
large lymphocytes, another neutrophilic leucocytes, and another 
eosinophilic leucocytes. Intravascularly, developing erythro- 
blasts predominate; among these are intermingled lymphoblasts, 
thromboblasts, and neutrophilic leucoblasts. 
Here again it might be argued in favor of the polyphyletic 
theory that since the general environment is apparently the 
same, the progenitors of the several groups were cells with dif- 
ferent and specific developmental potencies, in spite of their 
apparent morphologic identity. But in view of the fact that 
relatively slight environmental differences apparently determine 
developmental differences in early ontogeny, as, for example, in 
the developing gut of the mammalian embryo where smooth 
muscle, connective tissue, and blood develop in the same re- 
stricted regions, it seems more in accord with histogenetic data 
to conclude that the several regions filled with different segre- 
gated groups of cells were under the influence of different meta- 
bolic (fundamentally perhaps only relational, both temporal 
and spatial) factors, which determined the specific type of 
development. - 
This brings us to the question of the development of the 
eosinophilic leucocytes. The red marrow of the frog offers an 
especially favorable material for the study of the developmental 
history of the eosinophilic granulocytes. Unequivocal histologic 
evidence accrues with respect to the debated questions regarding 
the origin of the cells, whether heteroplastic or homoplastic; 
source of the eosinophilic granules, whether endogenous or exog- 
enous; the alleged basophilic ‘unripe’ eosinophilic granules, and 
the genetic relationship between mast-cells and eosinophilic 
leucocytes. An enormous literature of conflicting opinion and 
