446 H. E. JORDAN 
technic. This tissue proved helpful for comparisons in the 
course of this study, but the hemopoietic sequences were worked 
out on the one specimen with the abundant red marrow. In 
the frog the process of blood-corpuscle formation is said to occur 
only in the early summer, at which time only the bones contain 
red marrow (Starling). The specimen under consideration had 
presumably simply anticipated his fellows in this process of con- 
verting the yellow into red marrow. 
The reticular stroma of the red marrow is packed centrally 
with fat cells, blood-vessels, and hemogenic vascular spaces 
(angiocysts), and covered peripherally by a layer of differentiat- 
ing leucocytes. Erythrocytes are seen only centrally within the 
vascular spaces. 
The development of fat cells from the mesenchymal stroma 
can be traced through a complete series of stages. The process, 
however, presents nothing new except that a certain number of 
practically adult fat cells contain two nuclei. Endothelium, 
erythrocytes, lymphocytes, and leucocytes can likewise be traced 
through a complete series of developmental stages back to the 
marrow mesenchyma. Hemopoiesis is an essentially similar 
process, involving the formation of ‘blood-islands,’ in yolk-sae 
and red marrow. The spindle cells arise only intravascularly 
as differentiation products from small lymphocytes and from 
endothelium. Endothelium may also produce hemoblasts sec- 
ondarily. 
It seems preferable to describe the developmental history of 
the blood-cells in the marrow in the following order: erythro- 
cytes, lymphocytes, eosinophilic leucocytes, basophilic leuco- 
cytes, neutrophilic leucocytes, and thrombocytes. 
1. The development of the erythrocytes. This history can be 
read in the smaller blood spaces of the central portions of the 
red marrow. The larger spaces and blood-vessels contain only 
more mature erythrocytes, mingled with the granular leucocytes 
of extravascular origin. The erythrocytogenic process passes 
from a stage of solid strands of hemoblasts (marrow ‘blood- 
islands’) to one of forming vessels in which the central cells are 
free mature erythrocytes and the peripheral cells represent 
