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ZOOLOGY: H. E. JORDAN 
stoutest endothelial hemoblast at this stage is in the typical resting 
condition. 
As regards the aortic cell clusters, the 5 mm. mongoose embryo shows 
admirably various early stages in their origin and development, and 
so furnishes the key to the interpretation of the later products. And 
the 12 day loggerhead turtle embryo shows besides, the peculiar intra- 
vascular encapsulated cell clusters, and the endothelial strands, recently 
noted also for a 12 mm. pig embryo by EmmeP in a footnote to his 
paper (p. 407) on aortic cell clusters in mammals; and the conditions 
in this respect also are such as appear to solve the mystery of their 
genetic significance. 
The aortic cell clusters in the mongoose embryo of from 5 to 7 mm. 
range from such as are composed of only a single cell to those composed 
of a score or more. Single cells or groups of two or three can be seen 
separating from the endotheHum at any point, even along the mid- 
dorsal line. Larger groups are found only in the ventral and ventro- 
lateral portions, frequently in more or less close relation to the mouths 
of the lateral mesonephric branches or the ventral intestinal rami. This 
proliferative activity of the aortic endothelium is present only in the 
abdominal portion of the aorta, approximately coextensive with the 
mesonephroi. Single endotheHal cells may round up and take on 
hemoblast features and separate from the wall in exactly the same 
manner as that by which the hemoblasts are derived from the endothe- 
lium of the yolk-sac vessels and in the per^cerebral vascular channels. 
The process is the same in the yolk sac and the embryo, and indicates 
a common hemogenic capacity of embryonic endothelium. 
The mongoose material shows also the initial stages in the formation 
of the larger cell clusters. Throughout the ventral half of the abdominal 
aorta, the endothelium at certain points appears to buckle into the 
lumen. This invaginated area may be more or less extensive, and 
may include a considerable portion or none of the subjacent mesen- 
ch3ane. The cause of the buckling remains obscure, though the sug- 
gestion lies close to hand that it may be related to the caudal shifting 
of the embryonic representatives of the ceHac, superior mesenteric and 
inferior mesenteric arteries; a process dependent in part at least upon 
the presence of a less rigid and less differentiated endothelium ventrally, 
permitting thus of an inequality of growth as between the ventral and 
dorsal walls or allowing for the formation of successively lower connect- 
ing vascular segments for the migrating definitive stems. 
The endotheHum seems to be lacking centrally underneath the cell 
clusters. This is explained by the fact that the larger clusters arise by 
