BLOOD AND BONE-MARROW OF FROG 445 
approximately longitudinal grooves; certain of these are so deep 
as to appear to completely divide the nucleus. Similar condi- 
tions were previously reported also in blood smears of certain 
turtles.1! Downey* questions the accuracy of this interpretation 
and inclines to regard all these phenomena as simply deep fur- 
rows. One could hardly base final conclusions regarding this 
point on smear preparations; the process of spreading the blood 
on the slide might very readily divide the already deeply con- 
stricted (grooved) nucleus. But similar appearances occur also 
in the marrow sections. However, here it could always be 
argued that in such cases the plane of section passed above the 
bottom of the groove, thus giving the deceptive appearance of a 
division of the nucleus. Against the interpretation that certain 
of the thrombocyte nuclei are actually split must be cited the 
fact that these cells in the circulating blood of the frog contain 
generally a more compact, more nearly spherical and ungrooved 
nucleus (figs. 27 to 32). If a certain groove of the nucleus of 
the immature thrombocyte of the marrow is conceived to lead to 
a direct division of the nucleus, then certain mature thrombo- 
cytes should be binucleated, which is apparently never actually 
the case in the frog. On the contrary, the mature nucleus seems 
to have lost the grooves of its immature condition, apparently 
through a process of dilatation of the nuclear vesicle, involving a 
change from oval to spherical form and an obliteration of the 
grooves by reason of a filling up of the nucleus. 
b. The histology of the red bone-marrow 
This description is based almost exclusively on the bone-mar- 
row of the shaft of the femurs of an adult specimen killed the 
middle of January. This particular specimen had an especially 
abundant red marrow in its femurs. Practically the entire shaft 
was packed with red marrow. Other specimens examined at 
the same time and during the following month, as also specimens 
from a new shipment in April, showed only small patches of red 
marrow in the shaft of the femur. Sections were made of this 
essentially yellow marrow and also stained according to Wright’s”® 
