ZOOLOGY: H. E. JORDAN 
151 
The study was approached by way of the yolk sac of the mongoose 
embryo. The endothehal origin of hemoblasts can here be readily 
demonstrated. These observations on the mongoose yolk-sac confirm 
my 16 previous findings regarding the hemogenic role of yolk-sac endo- 
thelium in the 10 mm. pig embryo. 
The second step involved a search for similar intraembryonic phenom- 
ena. It seemed reasonable to expect that, since the yolk-sac mesen- 
chyme could differentiate directly into blood cells and into endothelium, 
and since the endothelium could subsequently transform into blood cells 
then the same order of events should probably follow also in the intra- 
embryonic mesenchyme; and further, since mesenchyme is the funda- 
mental hemogenic tissue, and since both endotheHum and mesothelium 
in the embryo are only sHghtly modified mesenchyme (chiefly by means 
of the mechanical factor of pressure — vide Huntington, p 265), 
then embryonic mesothelium and endothelium should both in the only 
sHghtly differentiated condition be capable of producing cellular blood 
elements (hemoblasts). 
That mesothelium can differentiate into vascular tissue has been 
shown by Bremer^^ in the case of the body stalk of a 1 mm. human em- 
bryo. Examination of the intraembryonic endothelium in the pig 
and mongoose revealed, in the smaller pericerebral blood channels, an 
occasional endotheHal cell rounding up and taking on hemoblast 
features and finally separating from the endothelial wall; and led to the 
discovery and detailed study of the aortic clusters of hemoblasts, with 
the origin and significance of which this paper is largely concerned. 
Moreover, investigation of the pericardial mesothelium disclosed very 
similar clusters, both attached to the visceral and the parietal peri- 
cardium, and lying free within the pericardial cavity. EmmeP^ has 
recently described comparable structures in the 12 mm. pig embryo. 
Occasional individual cells can also be seen in process of separation 
from the visceral pericardium in the mongoose embryo. 
The suggestion has been made that what is interpreted as a hemo- 
blast in the act of differentiation and separation from the endothelium, 
is simply an endothelial cell in preparation for division. Many divid- 
ing endotheHal cells from the pericerebral mesenchyme were observed 
with this suggestion in mind. But no endothelial cell, even at metaphase 
of mitosis, appeared rounded up in the same fashion as the differen- 
tiating hemoblast, nor to the same degree; at most it was merely a stout 
fusiform body, without the distinct unilateral bulge equatorially which 
is chaj-acteristic of the hemoblast during the later stages of its sep- 
aration from the endotheHum. Moreover, the nucleus of even the 
