ABBREVIATIONS 



ac, aortic cluster la, lateral aortic ramus 



d, mitosis to, the more highly differentiated baso- 



philic cells (or macrophags) in the 

 da, degenerating aortic ramus aortic clusters 



dw, dorsal aortic wall r, erythrocyte 



e, endothelium s, mesenchyma 



iam, intra-arterial cell mass va, ventral aortic ramus 



in, phagocytic inclusion vw, ventral wall of the aorta 



The direction of circulation and the long axis of the aorta is indicated by 

 an arrow. 



A portion of the ventral region of the aorta as seen in longitudinal section, 

 showing the general appearance and morphological relations of the aortic clus- 

 ters (aci-acs). A caudalward projection of the free end of the clusters is illus- 

 trated in ac4. Cluster acs is situated at the entrance to a root of the superior 

 mesenteric artery, da is a section of a degenerating aortic ramus packed with 

 basophilic cells similar to those of the clusters. 



2 and 3 Show the form and structural relations of the aortic clusters as seen 

 in transverse sections of the aorta. Adjacent to the base of these clusters may 

 also be observed a variation in the general arrangement of the mesenchymal 

 cells (s). 



4 Illustrates the double aortic clusters occasionally found. . 



5 One of two spherical masses of cells found in a 12 mm. embryo. The mass 

 appears in some respects comparable to the aortic clusters, but is surrounded 

 by a more or less definite endothelial membrane (en). Situated in the ventral 

 wall of the aorta. 



6 Section of an aortic cell cluster situated at the extrance to a ventral aortic 

 ramus and in intimate relationship with the intra-arterial cell mass {iam) fill- 

 ing the latter vessel. Also illustrates the more definite differentiation of the 

 peripheral cells (toi) of the clusters, some of which appear entirely detached from 

 the main mass (TO2). 



7 Section of an intra-arterial cell mass in a degenerating ventral aortic 

 ramus. Note absence of a definite lining endothelium and the evidence of active 

 cell multiplication (c). 



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