404 V. E. EMMEL 



is evidently one of greater significance than that of merely 

 incidental cellular accumulations. 



First, the clusters are in many cases at least, evidently rather 

 firmly attached to the aortic wall. This is indicated by the 

 fact that these structures may be found in the aorta even though 

 the vessel is practically empty as may occur for example through 

 the loss of blood during the preparation of the material. Again 

 in some cases in which during the process of fixation almost the 

 entire blood content settles toward one side of the aorta, the 

 clusters instead of being carried along with the rest of the blood, 

 continue attached in what appears to be their original position 

 on the aortic wall. Finally, it may be observed that many 

 of the larger cell masses present a rather elongated shape (fig, 1) 

 with one end of the long axis adherent to the aortic wall and 

 the other end free and directed caudalward (towards the left 

 in the figure); i.e., the relations are such as might be expected 

 in an attached cell mass one end of which was free to be carried 

 down stream by the force of the blood current. 



Second, the clusters are of constant occurrence in 6 mm. to 

 15 mm. embryos. Thus in a count for four 12 mm. embryos 

 there was a total of 45 clusters distributed in the proportion 

 of 11, 9, 13 and 12. On the other hand in embryos beyond about 

 the 15 mm. stage, these cell masses are absent. Such condi- 

 tions do not appear readily accounted for on the basis of acci- 

 dental agglutination. 



Third, in all the embryos studied the clusters were confined 

 to the ventral half of the aortic tube and with greatest frequency 

 toward the median region of its ventral wall (figs. 2, 3 and 4). 

 In no case was a cluster found in the dorsal aortic wall. Such 

 facts certainly appear indicative of a deeper relationship be- 

 tween these cell structures and the aortic tube. 



In connection with this conclusion it is of course to be recog- 

 nized that an oc.casional cell from the circulating blood may 

 possibly now and then have been fixed in such a manner by the 

 killing reagents as to be adherent to the vascular surface. On 

 the other hand sections other than these passing through the 

 base of the cluster may also present the deceptive appearance 



