BLOOD AND BONE-MARROW OF FROG 465 
granules, and the nucleus of the mast-cells is at all stages after 
the earliest quite different from that of the eosinophils. The 
presence of these cells in the smear preparations in considerable 
numbers renders inadmissable their interpretation in terms of a 
disintegration of other types of cells. The deep blue-staining 
granules of the basophilic myelocytes change to a violet color as 
seen in blood smears. The nucleus also changes from a deep- 
staining homogeneous body to a vesicular body with a delicate 
reticulum. It maintains an approximately central position in 
this cell throughout its history. 
The most perplexing matter regarding the thrombocytes con- 
cerns their origin. In a study of hemopoiesis in the turtle, 
Jordan and Flippin" have described their origin in part from the 
endothelium of. the original vascular spaces of yolk-sac and red 
marrow and in part from small lymphocytes. Danchakoff? 
claims that they arise exclusively from lymphocytes (throm- 
boblasts), while Werzberg?> maintains that they are a type sui 
generis, having origin neither from endothelium nor lympho- 
cytes. The evidence from this study of the red marrow of the 
frog confirms our former conclusion regarding their origin, as 
described in a study of turtle embryos. The thrombocytes 
arise only intravascularly. They can be traced directly to 
small lymphocyte-like cells. Secondarily, both lymphocytes 
and thrombocytes may arise from endothelium. The close 
relationship of small lymphocytes and thrombocytes appears 
throughout their entire history. This is especially emphasized, 
aside from the close similarity in structure of the nucleus, by the 
presence in certain of the small lymphocytes of a variable quan- 
tity of metachromatic granules. 
The mode of multiplication of the constituent cells of this 
marrow centers interest upon amitosis and throws additional 
light on the question of this variety of cell division. The orig- 
inal blood-cell progenitor (lymphocyte; hemoblast) arises from a 
mesenchymal cell. Relatively little proliferative activity occurs 
among these mesenchymal cells; the multiplication of hemoblasts 
results mainly from a division of already differentiated mesen- 
chymal cells. This differentiation process consists essentially in 
