458 H. E. JORDAN 
one important respect. Maximow claims that in Rana tempo- 
raria lymphocytes arise only in the blood-vessels. He notes 
that this is a significant departure from what occurs in the case ° 
of selachians, birds, and mammals, where the lymphocytes form 
both extra and intravascularly, and he suggests that this differ- 
ence may inhere in ontogenetic differences associated with holo- 
blastic and meroblastic cleavage. My preparations of the 
marrow of Rana pipiens show, however, that in this species 
lymphocytes arise in both locations. If a difference actually 
exists in the case of Rana temporaria it must be a specific dif- 
ference or a chance variation in Maximow’s specimens and not 
one characteristic of amphibia as a group. Maximow” further 
calls attention to the fact that in amphibia the lymphoid and 
myeloid tissues are not sharply separated topographically, there 
being here no sharply defined lymphoid organs with the specific 
function of lymphocyte production, like the lymph nodes of mam- 
mals, and another tissue with specific granulopoietic function. 
This mingling of lymphoid and myeloid tissues and functions in 
the bone-marrow gives additional support to the monophyletic 
theory. 
The erythrocyte, lymphocyte, and leucocyte series in common, 
can be traced through gradual steps of development from a type 
of cell which is indistinguishable in the several series, namely, a 
lymphocyte-like cell, the ‘hemoblast.’ The determining differ- 
entiation factor seems to be exclusively environmental. ‘Lym- 
phocytes’ that become enclosed by endothelium differentiate 
into erythrocytes, or in small part they may proliferate or per- 
sist as lymphocytes which undergo slight differentiation in pass- 
ing into the blood stream, or some may differentiate into 
thrombocytes. Furthermore, the already slightly differentiated 
endothelium may to some extent during its younger stages further 
differentiate into erythroblasts and into thromboblasts. Con- 
sideration of the common origin of the endothelium and the 
hemoblasts, from the original medullary ‘blood-islands,’ makes 
such differentiation intelligible. Neutrophilic leucocytes may 
also to some extent differentiate from intravascular hemoblasts 
and from circulatory lymphocytes. But extravascular condi- 
