330 SENESCENCE AND REJUVENESCENCE 



amphibia, and reptiles. More than thirty years ago Nussbaum 

 ('80) described the early differentiation of the sex cells in fishes 

 and amphibia. Later, Eigenmann ('92, '960) described the early 

 segregation of germ cells in fishes and found that the primitive 

 germ cells in the fish Cymatogaster were segregated in the fifth cell 

 generation of cleavage, and Wheeler ('oo) found a relatively early 

 differentiation of the germ cells in the lamprey. Two years later 

 Beard ('02), as the result of his work on selachians, reached the 

 conclusion that the germ cells are independent unicellular organisms 

 which pass a part of their life in the multicellular sterile soma. 

 This conclusion rests on the occurrence in embryonic stages of 

 certain large cells seen by various investigators in certain regions 

 of the embryo and which are described as migrating to the position 

 of the sexual organs and later becoming the germ cells. Since 

 Beard's paper, a large number of similar observations have been 

 made by various authors on fishes, amphibia, and reptiles. 



As regards all these data on germ-cell segregation in the verte- 

 brates, the first question is the correctness of the observations. 

 Much time has been devoted to the observation of these cells in 

 the embryonic stages and but little to the details of their later fate. 

 Moreover, the extensive migrations described from various regions 

 of the embryo to the position of the sexual organs are in all cases 

 inferences from the examination of fixed and stained material. 

 But granting that the observations are correct, the segregation of 

 the germ cells is no earlier in most cases than that of many other 

 parts of the body, and such cases afford no valid evidence against 

 the view that the germ cells are integral, specialized parts of 

 the body like other organs. In most cases these early germ 

 cells in vertebrates are, like those of invertebrates, apparently 

 cells with a lower rate of metabolism than other parts of the 

 embryo. Often they retain yolk granules later than other cells^ 

 and in all respects appear to be less active during early stages 

 (Eigenmann, '966). 



At present the only conclusion possible from all these observa- 

 tions on germ paths and germinal segregation is that while the data, 

 if correct, as they probably are in at least many cases, do indicate 

 that in various forms the germ cells become more or less distinctly 



