172 GERM-CELL CYCLE IN ANIMALS 



dial germ cells of other animals, but is unable to ar- 

 rive at any final conclusion. 



In certain CLADOCERA and COPEPODA, as we have 

 seen, there are visible substances within the cyto- 

 plasm of the egg which become segregated in, and 

 render distinguishable, the primordial germ cell. Some 

 species belonging to these and other groups of CRUS- 

 TACEA have been studied in which such a visible sub- 

 stance peculiar to the primordial germ cell is absent. 



Samassa (1893) not only failed to find the pri- 

 mordial germ cell during the cleavage stages of 

 Moina rectirostris, but claims that the germ cells 

 arise from four mesoderm cells. Kiihn (1908), from 

 a study of the parthenogenetic generation of Daphnia 

 pulex and Polyphemus pediculus, also derives the germ 

 cells from the mesoderm. Vollmer (1912) could not 

 distinguish the germ cells of Daphnia magna and D. 

 pulex in the developing winter eggs until the blasto- 

 derm was almost completed and Miiller-Cale (1913) 

 could not find these cells in Cypris incongruens until 

 the germ layers were fully formed. McClendon 

 (1906a) has shown that in two parasitic copepods, 

 Pandarus sinuatus and an unnamed species, the pri- 

 mordial germ cell is established at the end of the 

 fifth cleavage (32-cell stage) instead of at the end of 

 the fourth as Haecker (1897) found in Cyclops. It 

 is suggested that this delay may be due to the large 

 amount of yolk present. The stem-cell from which 

 it arises is, however, not made visibly different from 

 the rest of the blastoderm by peculiar granules as is 

 the case in Cyclops. 



