KEIMBAHN-DETERMINANTS 227 



posed regarding the role of these bodies in heredity 

 make it necessary to refer to them briefly here. At 

 the present time it is difficult to make any definite 

 statement regarding the origin, nature, and signifi- 

 cance of the various cytoplasmic inclusions that have 

 been grouped under the general title of mitochondria. 

 It seems probable that we are concerned with a num- 

 ber of different sorts of inclusions, and with various 

 stages in their evolution. In the guinea pig (Ru- 

 baschkin, 1910, 1912) and chick (Tschaschkin, 1910) 

 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 

 (Fig. 31, J5). Swift (1914) has found, however, 

 that in the chick the mitochondria in the primordial 

 germ cells are not at all characteristic, resembling 

 those of the somatic cells. The germ cells neverthe- 

 less can be distinguished from the latter by the pres- 

 ence of an especially large attraction-sphere (Fig. 

 31, D). This distinction between the primordial 

 germ cells and the surrounding somatic cells may 

 enable us to trace the keimbahn in vertebrates back 

 into cleavage stages something that has not been 

 accomplished as yet. 



An examination of the various keimbahn-deter- 

 minants listed in the table (p. 212) has led the writer 

 to conclude that none of them is of a mitochondrial 

 nature, but the results obtained by the special methods 



