GERM-CELL HISTORY IN THE BROOK LAMPREY 35 



that in the frog (Rana esculenta) all the embryos produced by 

 eggs in which fertilization had been delayed for a certain number 

 of hours were males. He believes that normally oogonia are 

 derived from primordial germ cells which are situated in the 

 germinal epithelium, while the spermatogonia are derived from 

 the axial mesenchyme. When fertilization is delayed no pri- 

 mordial germ cells are produced and all the definitive germ cells 

 come from mesodermal cells which he calls 'Paragonien.' These 

 give rise only to spermatogonia, and the larvae are therefore 

 males. Witschi ('14), who more recently has worked on the 

 development of the germ cells in Rana temporaria, finds that in 

 this form there is no secondary origin of germ cells. He says: 



So scheinen alle Tatsachen dafiir zu sprechen, dass von ihrem friih- 

 esten Erscheinen an, die Keimzellen als Gebilde spezifischer Natur zu 

 betrachton sind, welche. wenigstens unter Bedingungen die von nor- 

 malen nicht sehr abweichen, weder sich in somatische Elemente 

 umwandeln, noch aus solchen durch Unwandlung entstehen konnen. 



Among others who have followed the history of the germ cells 

 and have found no evidence of secondary origin, may be men- 

 tioned King ('08) for Bufo and Swift ('14, '16) for the chick. 



b. Method of migration of the germ cells. Three opinions 

 have been advanced concerning the method by which germ cells 

 reach the germ-gland anlage in vertebrates: 1. Most investiga- 

 tors are inclined to the belief that, by some sort of ameboid move- 

 ment, there is an active migration of the germ cells from the ento- 

 derm to the splanchnic mesoderm, and then through the mesentery 

 into the germ-gland anlage. That such migration exists seems 

 certain in forms in which the germ cells are so late in arriving at 

 their final destination that they lose their yolk before leaving 

 the entoderm. 



2. A second theory is, that in some of the lower forms the 

 migration of the germ cells may be accounted for partly by a 

 shift in the position of the somatic tissues around them. In this 

 case the germ cells are considered passive elements which take 

 little or no active part in the migration. 



It seem likely that in some forms migration is partly active 

 and partly passive. This is probably true in the lamprey. No 



