REPRODUCTION. 25 



with one another, and surrounded by a shell, often of the 

 most complicated description. In this case, however, the 

 buds produced by the primitive spherule are not only not 

 detached, but they can only remotely be regarded as independ- 

 ent beings. They are, in all respects, identical with the prim- 

 ordial segment, and it is rather a case of " vegetative " repeti- 

 tion of similar parts. 



Another form of gemmation is exhibited in such an organ- 

 ism 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 appearance. This colony is 

 produced by gemmation from a single primitive being (" poly- 

 pide "), which throws out buds, each of which repeats the pro- 

 cess, apparently almost indefinitely. All the buds remain in 

 contact and connected with one another, but each is, neverthe- 

 less, a distinct and independent being, capable of performing 

 all the functions of life. In this case, therefore, each one of 

 the innumerable buds becomes an independent being, similar 

 to, though not detached from, the organism which gave it 

 birth. This is an instance of what is called " continuous gem- 

 mation." 



In other cases as in the common fresh-water polype or 

 Hydra the buds which are thrown out by the primitive or- 

 ganism become developed into creatures exactly resembling 

 the parent, but, instead of remaining permanently attached, 

 and thus giving rise to a compound organism, they are de- 

 tached to lead an entirely independent existence. This is a 

 simple instance of what is termed " discontinuous gemmation." 



The method and results of fission may be regarded as essen- 

 tially the same as in the case of gemmation. The products of 

 the division of the body of the primitive organism may either 

 remain undetached, when they will give rise to a composite 

 structure (as in many corals), or they may be thrown off and 

 live an independent existence (as in some of the Hydrozoa). 



We are now in a position to understand what is meant, 

 strictly speaking, by the term " individual." In zoological lan- 

 guage, an individual is defined as " equal to the total result of the 

 development of a single ovum" Amongst the higher animals 

 there is no difficulty about this, for each ovum gives rise to no 

 more than one single being, which is incapable of repeating it- 

 self in any other way than by the production of another ovum; 

 so that an individual is a single animal. It is most import- 

 ant, however, to comprehend that this is not necessarily or 

 always the case. In such an organism as the sea-mat, the 



