ZOOLOGICAL SCIENCE 201 



constitute the subject-matter of embryology, are not 

 the only progressive modifications, occurring within the life 

 of the many-celled animal. There remain the minor changes 

 of adult life, and the degenerative changes immediately pre- 

 ceding the natural death of the individual. Here again, 

 we find problems that must be attacked experimentally. 

 Death occurs by accident in the vast majority of animals. 

 In nature, only the merest fraction of any generation lives 

 to grow up. The individuals, which live to grow old, are 

 frequently killed by their enemies before natural death can 

 intervene. When accidental death does not occur, natural 

 death is, seemingly, the inevitable fate of the individual 

 among the multicellular animals. 13 But this natural death 

 does not come to every cell encompassed by the body. 

 Certain of the germ-cells continue to live, through their 

 descendants which constitute the next generation. A 

 majority of the germ-cells perish, while a small minority of 

 them survives, if the race continues to exist. Hence, the 

 germ-cells are potentially immortal, while the body-cells 

 are destined to perish. 



It is perhaps worth while to inquire why one type of cell is 

 thus able under certain conditions, namely, union in fertili- 

 zation with another germ-cell, to continue its existence to 

 another generation, and so perhaps to all future generations. 

 The neighboring cells of the body are destined for old age and 

 death. Why this difference between germ and body-cell? 

 Save for cases of normal parthenogenesis, the germ-cells 

 die, if they do not unite in fertilization. Continuation of 

 their life hinges upon this one small matter of union with 

 another cell. The balance between death and life is so 

 slight that, in some of the experiments in artificial partheno- 



13 In animals which reproduce by budding or fission, it may be that the 

 individual can live on, as do the germ-cells. This has not yet been proved 

 experimentally for any many-celled animal; but plants like the potato or the 

 begonia may be reproduced indefinitely from cuttings, and, theoretically, an 

 animal like a hydra or a planarian might continue budding or fission forever, 

 and thus continue living without old age or sexual reproduction. 



