BLOOD AND BONE-MARROW OF FROG 447 
various stages of the differentiation from original hemoblasts. 
A certain number of the latter are in intimate connection with 
the superficially differentiating endothelium. Endothelial cells 
and blood-cells have differentiated from the same primitive cell 
mass, and subsequently for a time young endothelium may give 
origin to hemoblasts, including thromboblasts. 
The original progenitor of the erythrocyte is a cell with a rela- 
tively large vesicular nucleus and a narrow shell of slightly baso- 
philic eytoplasm. The nucleus is granular in character (figs. 37 
and 38). At first irregular in shape, i.e., polyhedral or even 
fusiform, the hemoblast soon assumes a spheroidal form. These 
cells proliferate mitotically. In becoming an erythrocyte the 
hemoblast (erythroblast) again changes into a stoutly oval form 
(fig. 41), the cytoplasm becomes oxyphilic as it elaborates hemo- 
globin, and the nucleus becomes more compact and more 
chromatic. 
The great difference in size between the red cells of the marrow 
and those of the circulating blood is surprising (compare figs. L 
and 41). Those of the smear preparations are approximately 
twice the size of those in the sections of the marrow. A certain 
large amount of this difference may be accounted for on the basis 
of a spreading out in the process of making the smear, but the 
residue can only be explained in terms of continued growth after 
leaving the marrow. The same phenomenon of postmedullary 
growth is evident in some degree in the case of all of the other 
types of blood-cells except the basophilic leucocytes (compare 
figs. 12 and 452). 
2. The development of the lymphocytes. As in the circulating 
blood, so in the marrow two main types of lymphocytes can be 
distinguished, differing in no essential nuclear or cytoplasmic 
features, but only in size (figs. 34 to 41). Transition forms 
occur abundantly. These intravascular lymphocytes are struc- 
turally indistinguishable from similar extravascular cells from 
which the granulocytes develop (compare figs. 88 and 41). Fur- 
thermore, the smaller varieties correspond with the hemoblasts 
from which erythrocytes, and in part thrombocytes, develop 
intravascularly. Intravascularly, the original hemoblast is more 
