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 Gonothyrea, 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 
