154 
ZOOLOGY: H. E. JORDAN 
The encapsulated cluster present in this same region of the inferior 
vena cava may be explained as follows: Subjacent to such clusters the 
mesenchyma appears to be differentiating into hemoblasts; this ob- 
servation may give the clue to the correct interpretation of these clusters. 
If the invaginating area of endothelium included a considerable por- 
tion of such differentiating (vascularising) mesenchyme, then the periph- 
eral cells might possibly be so far outstripped in the expression of 
their hemogenic potentiality as to be forced, perhaps principally by 
reason of internal pressure from the differentiating and proliferating 
cells, to continue development along the line already begun, namely 
into definitive endothelium. 
The whole series of phenomena above described seems to me to signify 
only various aspects of the same process, the hemogenic activity of 
embryonic endothehum. Though perhaps not essential, it appears 
nevertheless to be a normal process, consequent to the inherent capacity 
of endothelium to produce hemoblasts. 
EmmeP'^^ interprets the endothelial and mesothelial desquamation 
products, both cells and clusters, in terms of the stimulative effect 
of a pathologic factor upon the endothelium; a toxin whose source 
is in the degenerating cells of atrophying redundant ventral aortic 
rami, and in degenerating erythrocytes in the serous cavities in the case 
of the mesothelia. 
That atrophying vascular stems are present at this stage, both in 
relation to the aorta, and the inferior vena cava, cannot be disputed. 
In the 7 mm. mongoose embryo the solid regressive ventral aortic stems 
are especially conspicuous. At least a portion of the caudal shifting 
of the three large aortic rami is due to a progressive atrophy of upper 
portions of a connecting net of vessels. But coincident with this phase 
of a regressive development among the upper roots, there may possibly 
be a new formation of lower roots. I incline to see the cause of cluster 
formation in the latter possibility rather than in the former fact. 
Great stress is laid by Emmel upon the structure of the atrophying 
rami. Some of these are occluded by intravascular collections of hemo- 
blast-like cells, both in the 10 mm. pig embryo and in the 7 mm, mon- 
goose embryo. With these intra-arterial cell masses some of the aortic 
clusters are intimately related. Emmel ascribes the presence of this 
intra-arterial mass to the stimulative action of a dilute toxin, presum- 
ably liberated by the regressive aortic branches. This explanation is 
suggested by an alleged comparable pathologic process where endothe- 
hum is believed by certain pathologists (e.g., Mallory^^) to be stimulated 
to the formation of 'endothelial leukocytes' ('large mononuclear leuko- 
