26 



PETER OKKELBERG 



TABLE 3 

 Outline of theories concerning the origin of germ cells in vertebrates. The various 

 stages in the development of the individual are represented by five vertical columns. 

 The dotted lines indicate the germ paths as conceived by the various theories. 

 The different germ layers are represented by horizontal lines. The proximity of 

 the germ paths to each of these lines indicates where the germ cells are supposed 

 to take their origin and where they may be found during the different stages of 

 development. In the sixth vertical column a brief summary of each theory is given 



Blaa todann 



Tlirao Germ Layers 



Eabryo 



/i 



Uas-entotlerD 



<c 



Theory of early 

 aagregatlon. 



Rubaschkin '12 

 and others. 



/Eetoderm 



Hee-entodarm 



<c 



Maaoderm 



Gernlnal epitheli- 

 um theory and Gono- 

 tome theory. 



T/aldeyer '70, 

 RtJokert '08, and 

 others. 



4 



V- / 



Ues-antoderB v 



Oogonla derived 

 from primordial 

 germ cells, sperm- 

 atogonia from 

 me soderm cells 

 (Paragonlaj . 



Kusohakewl tsch 

 •10. 



< 



Haa-entoderm 



<c 



Primordial germ 

 cells degenerate- 

 secondary germ 

 cells derived from 

 mes.oderm. 



Von Winiwarter 

 and Satnmont '09. 



Hes-entoderm 



<c 



Primordial germ 

 cells may form 

 definitive germ 

 cells ^ut these 

 may also come from 

 mesoderm cells. 



Von Berenberg- 

 Gossler ' 14. 



'Ectodam 

 ^ 



Ues-antoderm 



< 



Primordial germ 

 cells may give rise 

 to a few oogonla 

 but most of the 

 definitive germ 

 cells are derived 

 from mesoderm cells. 

 Flrkat "14. 



