IO8 VERA DANCHAKOFF AND S. M. SEIDLIN. 



"cellules rhagiocrines " of Renaut and the clasmatocytes of Ran- 

 vier are endowed with phagocytic power. The polyblasts of 

 Maximoff, derived from the small lymphocytes, are very active 

 in inflammation processes. 



No efforts have been spared to find structural peculiarities 

 characterizing- the mononuclear macrophages to the exclusion of 

 lymphocytes. An ingenious method of identifying the macro- 

 phages was devised by H. Evans and elaborated by M. Simpson by 

 staining a characteristic set of granules numerous in the mononu- 

 clear cells and very scarce in the lymphocytes. 



That the class of macrophages, whether or not the mononuclear 

 leucocyte of the blood stream belongs to it, are well-defined struc- 

 tural units, is not a disputed fact, nor is their phagocytic ac- 

 tivity doubted. Their origin in the venous sinuses of the spleen, 

 lymph nodes and bone marrow, in the region where endothelium 

 and mesenchyme gradually merge into each other has been ad- 

 mitted. But do these well-established facts make the existence 

 of a much more extended digestive activity in mesenchyme and 

 its derivatives incredible in a multicellular organism? And why 

 should digestive activity be necessarily limited to macrophages of 

 serous cavities and to other cells identical with them as to their 

 structure and origin? 



A study of the conditions as they develop gradually in the em- 

 bryonic organism will allow of an easy realization of the fact, 

 that digestive activity is an inherent attribute of every cell, and 

 that it is retained by that part of the mesoderm which remains 

 the least differentiated in the form of mesenchyme. True that in 

 vertebrates the ectodermal and mesodermal layers are soon sepa- 

 rated from the yolk by the entoderm which remains in contact with 

 the yolk and develops into a series of highly specialized digestive 

 organs. But it is equally true that in earlier stages of develop- 

 ment the cells of the primitive streak are in direct contact with 

 the yolk and that the cytoplasm of all of them invariably contain 

 yolk granules, which gradually disappear by intracellular diges- 

 tion. All of the mesodermic phagocytes would derive their diges- 

 tive power from the cells of the primitive streak, being their 

 direct descendents. Actively manifested by the cells of the meso- 



