488 ROBERT W. HEGNER 



US hopes of tracing the germ-cell cycle back to cleavage stages. 

 A peculiar rod has been found in the spermatogonia of man which 

 is regarded as a Sertoli-cell-determinant (figs. 23-25, pp. 449-456). 



16. Metanucleoli and granules of various kinds have been de- 

 scribed in the mature eggs and early cleavage stages of a number 

 of animals, which may, upon closer study, be found to play some 

 role in the segregation of the germ cells (fig. 26, pp. 456-460). 



17. A table is given (p. 461) of the principal described cases 

 of the occurrence of the Keimbahn-determinants in animals. 

 These bodies in certain species arise from nuclear substances, 

 such as metanucleoli, chromidia, or the 'achromatishen Kern- 

 korper' (pp. 462-469). 



18. Yolk globules are characteristic of the primordial germ 

 cells and may play some role in their genesis ; nurse cells are known 

 to become Keimbahn-determmants in some Crustacea ; and 

 in certain other Crustacea the Keimbahn-determinants are 

 considered metabolic products; the mitochondria of germ cells 

 differ from those of the surrounding somatic cells (pp. 469-474). 



19. Observations and experiments indicate that the egg is 

 definitely organized and that this organization is continuous 

 throughout the germ cell cycle. The Keimbahn-determinants 

 are bodies which enable us to determine the position of that 

 part of the egg substance which controls the production of the 

 primordial germ cells, and to identify the stem cell and ulti- 

 mately the primordial germ cells when they are definitely es- 

 tabhshed (pp. 474-478). 



20. Keimbahn-determinants become visible at a definite time 

 and place, usually just before or just after maturation takes 

 place and cleavage begins. The localization of these bodies 

 is determined by the organization of the cytoplasm and takes 

 place during cleavage, either under the influence of the centro- 

 somes, or, more probably, by rearrangements of the egg contents 

 (pp. 478-482). 



21. The Keimbahn-determinants become distributed ap- 

 parently equally (except in Sagitta) between the daughter cells 

 of the primordial germ cell. As a rule they then gradually 

 disintegrate and disappear, and hence cannot be traced throughout 



