CELL CLUSTERS IN MAMMALL^N EMBRYOS 411 



to the atrophied remnant of a small artery which soon ter- 

 minates blindly in the adjacent mesenchyma. (Such depres- 

 sions are inadequately shown in ac2 and ac4 of figure 1, but can 

 be readily demonstrated in serial sections.) Not infrequently 

 in instances where such depressions are lacking there may still 

 be observed a clearly evident irregularity, sometimes of a more 

 or less whorled character, in the arrangement ofthe mesenchymal 

 cells at the base of the cluster as compared with the adjacent 

 regions of the aortic wall (figs. 2, 3, s). Occasionally the clus- 

 ters occur in pairs (fig. 4) as if they had arisen in connection 

 with the simultaneous atrophy of two paired aortic rami. In 

 apparent corroboration of these results certain conditions are 

 occasionally found at the aortic entrance to an as yet relatively 

 intact arterial ramus in which the cytological structure, form 

 and relations of the component elements of the vascular sur- 

 face suggest an early stage in the endothelial activities involved 

 in the production of the intra-arterial and aortic cell masses 

 (fig. 8). 



On the basis of the preceding data the conclusion is drawn 

 that the formation of the cell clusters in the aorta are not onlj^ 

 intimately associated with, but are also • evident^ correctly 

 interpreted as a direct result of the developmental processes 

 involved in the atrophy of the ventral and lateral aortiq rami 

 and the establishment of the permanent intestinal arteries of 

 the adult organism. ^ 



' Concerning the remarkable caudal wandering of the visceral rami of the 

 aorta a number of hypotheses have been advanced to account for the phenomenon 

 (Broman, '08, Tandler, ('93), Evans, '12) but the exact manner in which the 

 process takes place has apparently as yet not been established. In connection 

 with the present study it may be observed thait the depressions in the aortic 

 wall and the relations of the atrophied arterial stems and the aortic clusters 

 are such as to suggest that arterial remnants of the former aortic rami are 

 ultimately incorporated into the aorta itself. Such additions and consequent 

 inequalities of growth in the ventral region of the a>3rtic wall as contrasted with 

 its dorsal portion may in a final solution of the problem be found to contribute 

 materially to the caudal shifting of the coeliac and mesenteric arteries. Evans 

 ('12, p. 649) does indeed express the opinion that a primary factor in these vas- 

 cular changes is an unequal growth of the dorsal and ventral walls of the aorta 

 but has not, so far as I am aware, elaborated the specific nature of the process. 



