CELL CLUSTERS IN MAMMALIAN EMBRYOS 407 



possible migration of free cells from the mesenchyma into the 

 clusters. 



Finally, it is important to note that not infrequently the aortic 

 clusters are in direct relationship or continuity with cell masses 

 situated within certain atrophying arterial branches of the 

 aorta. These intra-arterial masses have evidently arisen in 

 situ from the endothelium of the artery in question and as 

 described elsewhere (pp. 409-411) probably constitute the pri- 

 mary source of origin of the aortic clusters. - 



III. CONCERNING THE CORRELATION OF THE CLUSTERS 

 WITH CERTAIN AORTIC DEVELOPMENTAL PROCESSES 



1. Degeneration and caudal wandering of aortic rami. In 

 the development of the mammalian aorta there occur two im- 

 portant vascular changes involving a shifting or caudal wander- 



- An additional observation which ma}' l)e conveniently recorded here relates 

 to a type of structure illustrated in figure 5. This group of cells is attached to 

 the ventral aortic wall and projects into the lumen of the vessel but differs from 

 the typical aortic cluster through its enclosure by a more or less definitely marked 

 peripheral membrane (en). In close relation to the membrane are a number 

 of cells some of which present the flattened endothelial form while others are 

 more rounded in shape. In other respects the component cells appear similar 

 to those of the clusters. Such structures are apparently of rare occurrence for 

 in the present matei'ial they were found in only one embryo, a 12 mm. specimen 

 (W. U. coll. No. 3), in which there w^ere two of these bodies, both ventrally lo- 

 cated. (This embryo had been stained with borax-carmine and the present 

 statements are made without having data derived from Giemsa stained material.) 

 It is of interest to observe that this same embryo was also deficient in the usual 

 number of aortic clusters, for only three of the latter were found instead of the 

 9-13 clusters in each of four other embryos of the same size. 



The aorta of the same embryo also contained three elongated cellular strands 

 evidently of endothelial nature. Two of these strands (about 60 micra in length) 

 were attached to the left umbilical artery near its origin from the aorta. The 

 third strand about 300 micra in length and varjang from one to several cells 

 in thickness, was connected by only a slender cytoplasmic strand to the aortic 

 wall. Two such structures were also found in a second 12 mm. embrj^o (W. U. 

 coll. No. 5), one in the aorta and the other in the region of origin of the umbilical 

 arteries. Nothing conclusive was ascertained as to the significance of these 

 strands, but the suggestion merits further investigation as to whether they may 

 possibly be associated with the fusion of the two original dorsal aortae. With 

 reference to our present purpose, however, it is of interest to note that many 

 of their component cells present rounded form, kidney shaped nuclei, and phago- 

 cytic activities comparable to that of the component cells of the aortic clusters. 



