BLOOD AND BONE-MARROW OF FROG 451 
eosinophils; basophils were not seen in division. A conspicuous 
and significant feature of these cells is the abundance and length 
of their granular pseudopods, certain of which may be seen seg- 
menting or fragmenting into faintly basophilic globules with 
granular centers, simulating thus very closely the blood-platelets 
of mammals in their structure and in their origin from mega- 
karyocytes (figs. 62 and 68). However, no naked nuclei could 
be found. The cells form common lyin the extra vascular tissue 
(fig. 69), but a few may possibly arise also from hemoblasts 
within the developing blood spaces of the marrow (fig. 41). 
6. The development of the thrombocytes. ‘These cells only arise 
intravascularly, from small lymphocytes, and in small part 
directly from endothelium. As they take on their definitive 
oval or fusiform shape they develop metachromatic granules. 
In their passage into the circulation they undergo further nuclear 
and slight dimensional changes as described above. They are 
never seen to arise extravascularly, nor do they undergo divi- 
sion; and the nucleus never assumes the crescentic lobulated 
condition characteristic of certain leucocytes. In the marrow 
they occur singly, in the blood smears generally in groups. The 
blood smears show thrombocytes with pseudopods, certain of 
which apparently constrict to produce platelets, as first described 
by Wright?§ for Amblystoma, leaving eventually disintegrating 
naked nuclei (figs. 31 and 32). 
7. The development of the plasma-cells and giant-cells. Certain 
large lymphocytes undergo a type of differentiation leading to 
typical plasma-cells. These plasma-cells are characterized by 
the coarse chromatic reticulum of their deep-staining nucleus, 
their irregular shape, and their very faintly basophilic, exten- 
sively vacuolated cytoplasm (fig. 71). This observation agrees 
with Downey’s® conclusion regarding the chief source of origin of 
the plasma-cells from lymphoid cells in the mesentery of Rana. 
_ A small number of lymphocytes undergo also a hypertrophy 
leading to mononuclear giant-cells. These cells contain a rela- 
tively enormous nucleus very like that of the younger lympho- 
cytes, and a variable shell of basophilic cytoplasm containing 
many fine metachromatic (lilac) granules (figs. 72 and 73). Con- 
