REPRODUCTION. 2$ 



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 pro- 

 duced by one individual (female) and the spermatic element 

 by another (male) ; in which case the sexes are said to be dis- 

 tinct, and the species is said to be "dioecious." 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 individual is said to be " her- 

 maphrodite," " androgynous," or "monoecious." In the case 

 of heraiaphrodite animals, however, self-fecundation- — contrary 

 to what might have been expected — rarely constitutes the re- 

 productive process ; 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 certainly does, occur, provisions seem to exist by 

 which perpetual self-fertilisation is prevented, and the influence 

 of another individual secured at intervals. Amongst the 

 higher animals sexual reproduction is the only process whereby 

 new individuals 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 generafive 

 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 modifica- 

 tions 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 permanently 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 



