CHAPTER XXI. 



REPRODUCTION. 



ALL physiological phenomena described in the previous chapters 

 have as their ultimate result the maintenance of the life of the 

 individual itself. No matter, however, with what regularity these 

 physiological processes are taking place, the life-period of a given 

 animal is not of an unlimited duration. Sooner or later the more 

 or less complicated mechanism stops its activity, and the individual 

 ceases to exist. With its death, not only all traces of the former 

 existence of the individual, hut also, with it, the existence of the 

 entire species to which the individual belonged, would be entirely 

 wiped out, had nature not provided for a process of rejuvenation, as 

 it were, of all living beings. This very important and peculiar phe- 

 nomenon in the economy of all living organisms, animal as well as 

 plant, is generally known as the process of Reproduction, the ulti- 

 mate aim of which is to maintain the species. We have to confine 

 ourselves to a consideration of animal reproduction only, and, even 

 here, we find this important process carried out in various ways. In 

 lower animals, in which a specialization of different parts of the body 

 to different functions is not yet established, the process of reproduc- 

 tion is also more or less simple. The body of an amoeba becomes 

 constricted and finally is divided in two halves, and each of these 

 halves becomes a fully developed animal, capable of multiplying in 

 the same fashion. A portion of a hydra, separated from the living 

 animal, is capable of developing into a complete new hydra. This 

 method of reproduction is called non-sexual. 



Throughout the greatest part of the animal kingdom, where we 

 find well-defined physiological division of labor in regard to other 

 vital functions, we also find the function of laying the foundation 

 for perpetuating the species assigned to special parts organs of 

 reproduction. The product of these specialized organs is known in 

 modern biology as the germ-plasm, and it is upon this structure that 

 the formation of a new individual and the transmission of all the 

 qualities from the parents to the offspring the heredity are con- 

 sidered to depend. The starting point for the development of every 

 individual we find represented in a typical cell; called an ovum, con- 

 taining the germ-plasm, and therefore also called germinal cell. 



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