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INTRODUCTORY REMARKS 
1. The analysis of the mechanism by which the male sex 
cell, the spermatozoon, causes the animal egg to develop is the 
subject of this book. This analysis was rendered possible 
through the fact that we are now able to imitate the action of 
the spermatozoon upon the egg by physicochemical means. 
The problem of fertilization is intimately connected with an- 
other problem, namely, that of natural death and immortality 
of the cell. Weismann enunciated the fact that the protozoa 
are immortal; and that the same immortality exists for the 
sex cells of metazoa.! 
The writer showed in 1902 that the problem of fertilization 
is intimately connected with the problem of the prolongation of 
the life of the egg cell. The unfertilized mature egg dies in a 
comparatively short time, which may vary from a few hours to a 
few weeks according to the species or the conditions under which 
theegglives. The fertilized egg, however, lives indefinitely, inas- 
much as it gives rise, not only to a new individual, but, theoreti- 
cally at least, to an endless series of generations. The death of 
the unfertilized egg is possibly the only clear case of natural 
death of a cell, i.e., of death which is not caused by external 
injuries, and the act of fertilization is thus far the only known 
means by which the natural death of a cell can be prevented. 
The two problems, fertilization and prolongation of life, are 
thus interwoven, and the! experiments on the mechanism of 
fertilization become at the same time studies on the problem 
of natural death and prolongation of the life of the egg cell.\ 
1 Leo Loeb pointed out ten years ago that his experiments on the transplanta- 
tion of cancer also proved the immortality of the cancer cell, since the same cancer 
cell tan be transplanted on endless generations of animals and live indefinitely. 
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