22 MANUAL OF ZOOLOGY. 



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

 heads. 



I. Sexual Reproduction. This consists essentially in the pro- 

 duction 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 spermatic element by another 

 (male) ; in which case the sexes are said to be distinct, and 

 the species is said to be ' dioecious.' In other cases the same 

 individual has the power of producing both the essential ele- 

 ments of reproduction ; in which case the sexes are said to be 

 united, and the individual is said to be 'hermaphrodite,' 'an- 

 drogynous,' or 'monoecious.' In the case of hermaphrodite 

 animals, however, self-fecundation contrary to what might 

 have been expected rarely constitutes the reproductive pro- 

 cess ; and as a rule the reciprocal union of two such individuals 

 is necessary for the production of young. Even amongst 

 hermaphrodite plants, where self- fecundation may, and cer- 

 tainly does, occur, provisions seem to exist by which perpetual 

 self- fertilisation is prevented, and the influence of another in- 

 dividual secured at intervals. Amongst the higher animals 

 sexual reproduction is the only process, whereby new indivi- 

 duals can be generated. 



II. Non-sexual Reproduction. Amongst the lower animals 

 fresh beings may be produced without the contact of an ovum 

 and a spermatozoid, that is to say without any true generative 

 act. The processes by which this is effected vary in different 

 animals, and are all spoken of as forms of 'asexual' or 'agamic' 

 reproduction. As we shall see, however, the true 'individual' 

 is very rarely produced otherwise than sexually, and most 

 forms of agamic reproduction are really modifications of 

 growth. 



a. Gemmation and Fission. Gemmation, or budding, con- 

 sists in the production of a bud, or buds, generally from the 

 exterior, but sometimes from the interior, of the body of an 

 animal, which buds are developed into independent beings, 

 which may or may not remain peraianently attached to the 

 parent organism. Fission differs from gemmation solely in 

 the fact that the new structures in the former case are pro- 

 duced by a division of the body of the original organism into 

 separate parts, which may remain in connection, or may under- 

 go detachment. 



The simplest form of gemmation, perhaps, is seen in the 

 power possessed by certain animals of reproducing parts of 

 their bodies which they may have lost. Thus, the Crustacea 



