REPRODUCTION. 33 



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 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 (" agamogenesis "). As we shall see, 

 however, the true " individual " is very rarely produced other- 

 wise than sexually, and most forms of agamic reproduction are 

 really modifications of growth. 



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

 sists in the produ.ctiQri_ja a bud, or buds, generally from the 

 exterior, TnTt 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. Fisskp 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 

 possess the power of reproducing a lost limb, by means of a 

 bud which is gradually developed till it assumes the form and 

 takes the place of the missing member. In these cases, how- 

 ever, the process is not in any way generative, and the pro- 

 duct of gemmation can in no sense be spoken of as a distinct 

 being (or zooid). 



An excellent example, however, of true gemmation is ex- 

 hibited in such an organism as the common sea-mat (Flustra), 

 which is a composite organism composed of a multitude of 

 similar beings, each of which inhabits a little chamber, or cell ; 

 the whole forming a structure not unlike a sea-weed in appear- 

 ance. This colony is produced by gemmation from a single 

 primitive being (" polypide "), which throws out buds, each of 

 which repeats the process, apparently almost indefinitely. All 

 the buds remain in contact and connected with one another, 

 but each is, nevertheless, a distinct and independent being, 



tion never occurs, but the plant is fertilised by the intervention of insects. 

 Thus, in many plants the stamens and pistil arrive at maturity at different 

 times, whilst in others the stamens and stigma are placed at different 

 heights in the flower, and do not always occupy the same position even in 

 a single species. 



C 



