298 Papers from the Department of Marine Biology. 



CELL-CLUSTERS OF THE AORTA. 



The next evidence for hemogenic capacity on the part of the endothe- 

 lium pertains to certain phenomena in the abdominal portion of the 

 dorsal aorta, the so-called "cell-clusters." Similar clusters in mam- 

 malian embryos were first (1909) observed by Maximow (ll) in the 

 rabbit embryo. 1 Dantschakoff (s) had already reported comparable 

 structures in the chick embryo. Minot (is) later (1912) described 

 them also for human embryos between 8 and 10 mm. Emmel (4) 

 subsequently (1915) studied more in detail these clusters in pig, rabbit, 

 and rat embryos. At the same time I was making a detailed study of 

 the aortic cell-clusters in the 10 to 12 mm. pig embryos, and had come 

 to essentially the same tentative conclusion regarding their significance 

 as Emmel, namely, that they were masses of hemoblasts differentiating 

 from the endothelium. In the 10-mm. pig embryo these clusters are 

 abundant and very large, some consisting of 100 or more cells; they 

 are practically limited to the ventral portion of the aorta (fig. 24). 

 Proximally they are in continuity with the endothelium and the sub- 

 jacent mesenchyma. Peripherally they consist of typical hemoblasts 

 and young erythroblasts. The former are characterized chiefly by a 

 deeper-staining basophilic cytoplasm, the latter by a lighter-staining 

 acidophilic cytoplasm. Centrally transition stages occur between endo- 

 thelium and hemoblast. Occasionally the clusters contain a central 

 core of only slightly differentiated endothelial cells. A number of the 

 cells may be in mitosis, and some exhibit phases in nuclear amitotic 

 division. The mongoose material is of special value in that it shows 

 the earlier stages in the formation of these clusters and thus gives 

 the key to their proper interpretation. The pig material showed 

 a progressive size increase between the 5 and 12 mm. stages of develop- 

 ment. The mongoose material shows the same thing. In the 7-mm. 

 embryo the clusters are somewhat larger and the peripheral cells more 

 differentiated than in the 5 mm. embryo. 



Figures 5 and 6 show typical small clusters, the first from the ventro- 

 lateral wall of the aorta, the latter from the mid-ventral line. Figure 

 5 gives the appearance of a buckling of the endothelium into the lumen 

 of the aorta, the peripheral cells of the invaginated area assuming 

 hemoblast characteristics. In the cluster, figure 6, the peripheral cells 

 have progressed still further along this line of differentiation, and the 

 proximal pole shows an increase of amitotic proliferative activity and 

 transition phenomena on the part of the endothelium. In the case of 

 the larger clusters the subjacent endothelium has in some instances 

 undergone considerable thickening, being frequently three layers thick. 



The close association between the cluster and the endothelium (figs. 

 25, 26, and 27) through a portion showing a transition between endothe- 



^rofessor Van der Stricht informs me that he had observed aortic cell-clusters in bat embryos 

 as early as 1899; but he claims that the cells of these clusters only differentiate into leucocytes. 



