88 GERM-CELL CYCLE IN ANIMALS 



engulfing other primitive ova. Only six or eight, 

 rarely more, of the eggs survive. 



In Clava, according to van Beneden (1874), the 

 ova arise in the entoderm. Weismann (1883) was 

 not able to determine whether they originated in 

 the entoderm or migrated into that layer from the 

 ectoderm, but he was certain that the male germ cells 

 were ectodermal. This conclusion regarding the 

 male germ cells was confirmed by Thallowitz (1885). 

 Harm (1902) was able to trace the primitive germ 

 cells back to a very early stage, and could distinguish 

 them in even young hydranths. The oocytes dif- 

 fered from the remaining ectoderm cells in the pos- 

 session of a larger amount of cytoplasm, a larger 

 nucleus with a big nucleolus, and an ameboid shape. 



Hargitt (1906), working on Clava leptostyla, comes 

 to conclusions different from those of Harm on C. 

 squamata. He says "that eggs probably never arise 

 in the ectoderm but always in the entoderm of the 

 peduncle of thegonophore, or in that of the polyp very 

 near the base of the gonophore. . . . Clava, like 

 other Hydroids, has its breeding season, during which 

 the germ cells are extremely abundant, and at other 

 times these cells are either entirely absent or very 

 scarce" (p. 208). Concerning the early origin 

 of germ cells Hargitt says, "it may not be im- 

 possible that * Urkeimzellen ' should perhaps exist in 

 undifferentiated stages, still the probability is so 

 extremely remote as to render doubtful to a degree 

 any but the most thoroughly substantial claims " 

 (p. 209). 



