GERM-CELL HISTORY IN THE BROOK LAMPREY 25 



epithelium which are descendants of the primordial germ cells; 

 while the spermatogonia are developed from 'Paragonien/ or 

 secondary germ cells, which take their origin in the axial mesen- 

 chyme. Von Winiwarter and Sainmont ('09) regard the pri- 

 mordial germ cells in mammals as only temporary structures, 

 which later degenerate. The same conclusion has been 

 reached by Kingery ('17). Von Berenberg-Gossler ('14), from 

 his work on Lacerta agilis, comes to the conclusion that the 

 migration of the so-called primordial germ cells from the ento- 

 derm is nothing but, ''eine spate, sich nach liingere Zeit hinzie- 

 hende Mesodermbildung aus dem Entoderm." He thinks that 

 these cells as well as other mesoderm cells, such as those of the 

 coelomic epithelium, may give rise to the stem cells of the ova 

 and spermatozoa. According to Gatenby ('16), there is in the 

 frog (Rana temporaria) and other amphibians an annual trans- 

 formation of peritoneal cells into germ cells, so that in these 

 forms there can be no talk of a continuity of the definitive germ 

 cells and the primordial germ cells. 



Some investigators believe that they have found evidence 

 that the primordial germ cells are segmentally arranged, being 

 derived from segmental portions of the mesoderm (Riickert, 

 Van Wijhe, Dustin, and others). This has been termed the 

 'gonotome theory.' It is an attempt to homologize the con- 

 dition found in vertebrates with that found in Amphioxus 

 in which the gonads are segmentally arranged from the begin- 

 ning of their development. According to this theory, the germ 

 cells are derived from mesodermal cells. 



Only a few investigators have followed the later history of the 

 primordial germ cells and found that they actually give rise to 

 definitive germ cells. Among these are Witschi ('14) for the frog, 

 King ('08) for the toad, and Swift ('14, '16) for the chick. Many 

 supporters of the theory of early segregation have studied only 

 the early embryonic stages and have assumed that definitive 

 germ cells originate from no other source than the primordial 

 germ cells. 



From the above it is clear that there exists at the present 

 time a great diversity of opinion concerning the origin of the 



