ZOOLOGY: H. E. JORDAN 
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an invagination of the endothelium over an area of some extent rather 
than by process of proliferation of one or several differentiating endo- 
thelial cells. Proximally the clusters show transition stages between 
endothelial cells and hemoblasts (laterally) and between mesenchymal 
cells and hemoblasts (centrally). The subjacent mesenchyme of the 
larger clusters may become thickened, sometimes assuming the features 
of a stratified endothelium, and the nuclei are relatively much more 
abundant, smaller, and less differentiated. In the larger clusters, the 
peripheral cells, some of which show early erythroblast features, begin 
to separate from the central group. Within the clusters some of the 
cells are in mitosis, while the nuclei of others may appear at some phase 
of amitotic division; and an occasional cell may show phagocytic proper- 
ties. Sometimes the core of the cluster shows transition stages between 
the endothelium or mesench3niie and hemoblasts. Many of the nuclei 
subjacent to the cluster appear at some phase of amitotic division. 
The absence of mitotic figures in the subjacent endothelium constituted 
the strongest of the three objections made by Minot^ against 
the interpretation of the aortic clusters as endothelial derivatives. 
It would seem that the method of proliferation is here largely amitotic. 
The aortic cell clusters of the mongoose embryo originate from the 
cells of an invaginated area of endothelium; they enlarge by intrinsic 
growth and differentiation, not by accretions from the circulating 
blood. Similar clusters appear also in the superior mesenteric artery. 
In a 10 mm. pig embryo a large aortic cluster, 130 microns in diameter, 
appears near the mouth of the superior mesenteric artery and consists 
of a hundred or more cells. Clusters appear also along the greater 
length of this definitive aortic stem. 
In the 12 day loggerhead turtle embryo, encapsulated clusters and 
extensive strings of hemoblasts attached to the endothelium appear in 
the inferior vena cava, near the point of fusion of the original paired 
subcardinal veins; and in the jugular veins. The endothelial strands, 
some of the cells of which bear hemoblast features, are most probably 
only another aspect of the general hemogenic capacity of young endo- 
thelium. EmmeP saw similar strands in the proximal portion of 
the left umbilical artery, and in the aorta of this level, in two 12 mm. 
pig embryos, and suggests that they may be related to the fusion of the 
two original dorsal aortae. In the case of the development of the in- 
ferior vena cava, the coincident fusion between the originally separate 
post — and sub-cardinal veins involves the formation of young, less dif- 
ferentiated, endothelium and so offers a favorable site for hemoblast 
production by endothelium. 
