406 V. E. EMMEL 



differentiation rather than cellular disintegration appears dem- 

 onstrated by the not infrequent evidence of mitotic activity 

 as well as phagocytic function in the component cells of the 

 clusters (fig. 9). 



Fifth, areas of the aortic wall are found in which the endo- 

 thelial cells present structural characteristics comparable to 

 those already described in the endothelium adjacent to the 

 larger cell masses (fig. 10a). Such areas are not, however, in 

 any direct juxtaposition to the aortic clusters nor are they to 

 be explained as deceptive appearances due to oblique or tangential 

 sections of the endothelial surface. It may be observed that the 

 endothelial cells are closer together, project above the general 

 level of the vascular surface, and not infrequently take a more- 

 basophilic stain. The nuclei may be either rounded in form or 

 approximate a kidney-shaped contour. No evidence was ob- 

 served of mitotic cavity indicative of merely an incidental in- 

 crease of endothelial cells in such regions through ordinary 

 endothelial growth and cell multiplication. It is also note- 

 worthy that such conditions were not found in the dorsal aortic 

 wall as is illustrated in a comparison of figures 10a and 10b. 

 Indeed the cytological structure and vascular relations of these 

 cells appear identical with that of cells occurring in the aortic 

 clusters. The fact that the aortic cell clusters present a great 

 difference in size varying from the large masses in figures, 2 6 

 and 9 to these smaller accumulations of only a few cells and that 

 they are also no longer present beyond certain stages of em- 

 bryonic development, suggests that such areas as shown in 

 figure 10a may represent end stages in the gradual dissociation 

 and final disappearance of the aortic clusters, in which, how- 

 ever, there still remain indications of the endothelial reaction 

 which has given rise to these structures. Sixth, of the two fixed 

 tissue elements of the aortic wall which could possibly take 

 part in the cell activities in question it is evidently primarily 

 the endothelium rather than the mesenchyma which partici- 

 pates in the formation of these cell masses. The demarcation 

 between the mesenchyma and the aortic clusters is fairly well 

 defined, nor was there obtained any conclusive evidence of a 



