342 AIMEE S. VANNEMAN 



germ cells in mammals. The question arises whether the germ 

 cells have been overlooked in the blood vessels, or whether 

 they never occur in the vascular system of mammals. It is 

 because of the before-mentioned distinctness of the germ cells 

 that the armadillo has been chosen and found peculiarly favorable 

 for this study. The species here used is Tatusia novemcincta. 



Although in recent years a large amount of work has been done 

 on the origin and history of the germ cells in various forms, yet 

 in vertebrates, relatively little has been accomplished along 

 this line. The question in this case is a far more difficult one, 

 as is acknowledged by all investigators who have attacked the 

 problem. The possibility of distinguishing germ cells from so- 

 matic cells in most vertebrates is rendered smaller by the absence 

 of characteristic yolk substances and further, by the appearance 

 only in advanced stages of the various so-called Keimbahn 

 determinants, which serve so admirably for the definite recog- 

 nition of the germ cells of certain invertebrates. Modern 

 advances in cytological methods, however, give promise of a solu- 

 tion to the problem, as is evidenced by the recent results of 

 Rubaschkin ('10), Tschaschin (10), Swift ('14) and others. 



It is unnecessary here to discuss the earlier literature of germ 

 cell history except to note that Eigenmann ('97) was the first 

 to give a detailed account of the wandering of germ cells in 

 vertebrates. He reached the conclusion that the sex cells in 

 fish are probably set aside as far back as the thirty-two cell 

 stage. This is the furtherest that any vertebrate germ cell 

 has been traced. 



Among those advocating the early origin of germ cells, may be 

 mentioned Beard ('00) in his work on Raja bates. He claims that 

 the germ cells may be found in a stage preceding the appearance 

 of any real embryo, the size of the cells suggesting origin after 

 about the thirteenth division. These, he finds, journey from 

 outside the embryonic region between the blastodermic layers 

 upwards through the yolk sac into the splanchnopleure and gut 

 regions. 



In 1902, Woods studying Acanthas, found that the germ cells 

 appear in the entoderm before the differentiation of mesoderm 



