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CHAPTER X. 



REPRODUCTION. 



REPRODUCTION is the process whereby new individuals are 

 generated, and the perpetuation of the species is insured in 

 spite of the constant deaths of its component members. 

 The modes in which this end may be attained exhibit a 

 good deal of diversity, but they may be all considered under 

 two heads. 



I. Sexual Reproduction. This consists essentially in the 

 production of two distinct "elements, a germ-cell or ovum, 

 and a sperm-cell or spermatozoid, by the contact of which 

 the ovum, now said to be "fecundated" is enabled to 

 develop itself into a new individual. As a rule, the germ- 

 cell is produced by one individual (female), and the sper- 

 matic element by another (male) ; in which case the sexes 

 are said to be distinct, and the species is said to be " dioe- 

 cious." In other cases the same individual has the power 

 of producing both the essential elements of reproduction ; 

 in which case the sexes are said to be united, and the indi- 

 vidual is said to be "hermaphrodite/' "androgynous," or 

 "monoecious." In the case of hermaphrodite animals, 

 however, self-fecundation contrary to what might have been 

 expected rarely constitutes the reproductive process ; and, 

 as a rule, the reciprocal union of two such individuals is 

 necessary for the production of young. Even amongst 



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