STUDIES ON GERM CELLS 441 



In certain Cladocera and Copepoda, as we have seen, there are 

 visible substances within the cytoplasm of the egg which becomes 

 segregated in, and render distinguishable, the primordial germ 

 cells. Some species belonging to these and other groups of 

 Crustacea have been studied in which such a visible substance 

 peculiar to the primordial germ cells is absent. 



Samassa ('93) not only failed to find the primordial germ cell 

 during the cleavage stages of Moina rectirostris, but claims that 

 the germ cells arise from four mesoderm cells. Kiihn ('08), 

 from a study of the parthenogenetic generation of Daphina pulex 

 and Polyphemus pediculus, also derives the germ cells from the 

 mesoderm. Vollmer ('12) could not distinguish the germ cells 

 of Daphnia magna and D. pulex in the developing winter eggs 

 until the blastoderm was almost completed and Muller-Cale 

 ('13) could not find these cells in Cypris incongruens until the 

 germ layers were fully formed. McClendon ('06) has shown 

 that in two parasitic copepods, Pandarus sinuatus and an un- 

 named species, the primordial germ cell is established at the end 

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

 fourth as Haecker ('97) 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. 



Bigelow ('02) has described in Lepas anatifera and L. fascicularis 

 certain stages which may bring the forms in which no early 

 segregation of the germ cells has been discovered into line with 

 the apparently more determinate species. In Lepas the yolk, 

 which at first is evenly distributed within the egg, passes to the 

 vegetative pole and becomes segregated in one of the first two 

 cleavage cells {cd"^). At the 16-cell stage the yolk lies within 

 the single entoblast cell {d^'^), which occupies a position corre- 

 sponding to that of the primordial germ cell in Moina. In this 

 connection may be mentioned the fact that in many animals the 

 germ cells are supposed to come from the entoderm and are char- 

 acterized by the possession of much yolk. 



