98 GERM-CELL CYCLE IN ANIMALS 



more or less dormant condition, just as they are in 

 more complex animals. Furthermore, the germ cells 

 must be widely scattered, as has been shown by Harm 

 (1902) in the young hydranths of Clava, by Wulfert 

 (1902) in the planula of Gonothyroea, and by Small- 

 wood (1909) in the polyp of Hydractinia. This wide 

 distribution of primitive germ cells accounts for the 

 reproductive powers of regenerated pieces of hy- 

 droids. 



3. Vertebrata 



Efforts have been made by many investigators to 

 trace the keimbahn in vertebrates, but thus far no 

 method has yet been devised which will enable us 

 to distinguish germ cells from other cells in the early 

 embryonic stages. That we shall be able to recognize 

 germ cells in still earlier stages of development than 

 has yet been accomplished seems certain, and the 

 recent contributions of Rubaschkin (1910), Tschasch- 

 kin (1910), von Berenberg-Gossler (1912a) and 

 Swift (1914) have already made considerable ad- 

 vances by the use of some of the more modern cyto- 

 logical methods. Three principal theories have been 

 advanced regarding the origin of the germ cells in 

 vertebrates, and these will be briefly stated before 

 the histories of the germ cells in special cases are 

 discussed. 



The germinal epithelium theory was advanced by 

 Waldeyer in 1870. At that time nothing was known 

 regarding the migration of germ cells during the 

 embryonic development of vertebrates, and it is 



