CHAPTER V 

 FERTILIZATION 



THE complex processes of the formation, differentiation, and 

 maturation of the germ cells, described in the two preceding 

 chapters, are to be regarded as preliminaries to the process of 

 fertilization. They can be understood only as preparatory 

 steps leading to the final meeting of an ovum and a spermato- 

 zoon, and their fusion into a single cell. The cell thus formed 

 is a "new" organism, which immediately commences a long 

 series of reactions, collectively termed development, leading 

 finally to the establishment of a form resembling that of the 

 individuals from which the fusing germ cells were themselves 

 derived. 



Among animals, with the probable exception of a few of the 

 simplest, an almost invariable condition of the continued ex- 

 istence of any specific form of protoplasm is such a periodic 

 mingling of the living substance of two individuals of the same 

 species. The few exceptions among the Metazoa are found in 

 the rare self-fertilizing hermaphroditic creatures, and even 

 here the mingled plasms may be said to have had somewhat 

 separate histories, although formed within the body of a single 

 organism. Animals which reproduce parthenogenetically, or 

 by such methods as budding or fission, sooner or later in their 

 life history exhibit these processes of germ cell formation and 

 fusion. 



Among the plants, on the other hand, while the union of 

 germ cells may be a frequent, and in some cases a necessary 

 preliminary to the formation of a new organism, yet other 

 tissues and living masses composed of several kinds of tissue, 

 taken from almost any part of the organism, may give rise to a 

 new individual. Many species of plants are thus normally 

 propagated by cuttings of leaf, stem, or root, or by runners, 



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