472 ROBERT W. HEGNER 



this has been done by Benda ('03) Prenant ('10), Faure-Fremiet 

 ('lO)i and especially Duesburg ('11). At the present time it is 

 difficult to make any definite statement regarding the origin, 

 nature, and significance of the various cytoplasmic inclusions that 

 have been grouped under the general title of mitochondia. It 

 seems probable that we are concerned with a number of different 

 sorts of inclusions, and with various stages in their evolution. 



Mitochondria appear to be present in practically all cells, 

 at least at some period in their existence. They have been ob- 

 served in plants as well as in animals, and in living as well as in 

 fixed and stained cells. Many terms have been applied to them 

 of which the most frequently employed are mitochondria (Benda, 

 '03), chondriosome (Meves, '08) plastochondria (Meves, '10) 

 and plastosomes (Meves, '10). The advocates of the chromidia 

 believe that these granules include mitochondrial formations 

 (Goldschmidt, '04) and that the latter are therefore of nuclear 

 origin, i.e., chromatic (Popoff, '07; Wassilieff, '07; Buchner, '10; 

 Jorgensen, '10; et al.). On the other hand, the majority of 

 investigators consider the mitochondria as cytoplasmic bodies 

 (Vejdovsky, '07; Meves, '08; Duesberg, '11; Wilke, '12;etal.). 

 Various functions have been ascribed to the mitochondria. 

 Benda considered them to be motile; Regaud ('09) thinks that 

 they fix and concentrate various substances in the cell (^fonction 

 electique' of Renaut); and Meves ('08) maintains that they 

 represent an important heredity substance, with the same 

 relation to the cytoplasm that the chromosomes have to the 

 nucleus. 



Of particular interest to us are the results of Rubaschkin 

 ('10, '12) and Tschaschin ('10) on the germ cells of vertebrates. 

 In the guinea-pig and chick the chondriosomes of the cleavage 

 cells are spherical and all similar, but, as development proceeds, 

 those of the cells which become differentiated to produce the 

 germ layers unite to form chains and threads, whereas those 

 of the primordial germ cells remain in a spherical and therefore 

 primitive condition (figs. 23, B; 24). This distinction between 

 the mitochondrial nature of the primordial germ cells and the 

 surrounding somatic cells may enable us to trace the Keimbahn 



