CELL CLUSTERS IN MAMMALIAN EMBRYOS 413 



Finally the^ experimental rcsultH of a number of investigators 

 of whom may be mentioned Tschaschin ('13, p. 370) and Mac 

 (.'urdy and Evans ('12, p. 1695) ,may be adequately sunmiarized 

 in the recent statement by Evans ('15, p. 254) "that occurrences 

 which place the endothelium of the most various vessels under 

 conditions, such, for instance, as a direct injury of the endothel- 

 lium, cessation of the adjacent current, in short in all cases of 

 thrombosis or embolism, lead to the proliferation of endothe- 

 lium . . . ." "Probably no area of the body can be ex- 

 cluded in this respect."^ 



In conclusion, therefore, it may be stated that while the original 

 assumption of the angioblast theory — that vascular endothelium 

 does not give rise to cellular elements of the blood — may under 

 normal conditions be true for the general systemic vascular 

 system, it appears that in both embryo and adult mammals, 

 endothehal tissue ordinarily passive may under certain abnormal 

 conditions, however, assume proliferative activities contributing 

 to the free cellular elements of the circulating blood. 



V. RESUME 



1. During the development of mouse, rabbit, and pig em- 

 bryos certain well defined cell masses or clusters are found in 

 the aofta of these mammals. 



2. The majority of the component cells of these clusters are 

 in their cytological characteristics comparable to the basophilic 

 and phagocytically active cells or macrophags (mesamoeboids?) 

 in the embryonic circulation. 



3. Their constancy of occurrence, firm attachment and restric- 

 tion to the ventral wall of the aorta indicate that these cell 

 clusters are not merely chance cellular accumulations but struc- 

 tures having a significant relationship to the vascular conditions 

 in the aortic artery. 



' The participation of the mesotheliiiiu in the origin of macrophags in the 

 embryonic coelom (Emmel, '15) is not improbably also a reaction to stimulative 

 conditions arising in part at least through degeneration and disintegration of 

 erythrocylic and other foreign elements escaping into these cavities. 



