8 EMBRYOLOGY. 



external gemmation. The organisms produced by gemmation 

 are not always similar at birth to the parent ; e.g. Acineta. 



Both fission and gemmation when incomplete lead to the 

 formation of colonies. 



The third mode of reproduction, by spore formation, does 

 not essentially differ from that by multiple fission. It consists 

 in the breaking up of the organisms into a number (usually very 

 considerable) of portions ; each of which eventually developes 

 into an organism like the parent form. All gradations between a 

 simultaneous division of the organism into such spores and simple 

 multiple fission are to be found, but this process of reproduction 

 may be sometimes distinguished from that by such fission by 

 the fact that the two processes may coexist in a single form, 

 e.g. the biflagellate monad of Drysdale and Dallinger. In the 

 majority of cases the spores produced differ at first from the 

 parent organism not only in size but in other points, such as the 

 possession of a flagellum, etc. They may even be without a 

 nucleus when the parent organism is nucleated, as in the Gre- 

 garinidae. 



The encystment, which in many cases precedes reproduction 

 by any of the above processes, and more especially by 

 spores, is not an essential condition of their occurrence ; and is 

 probably in the first instance a protective arrangement which 

 has become secondarily adapted to and connected with re- 

 production. 



As has been already stated, all the above modes of reproduc- 

 tion take place in some of the Protozoa without any anterior 

 process which can be regarded as of a sexual nature ; but very 

 often they are preceded by the temporary or permanent fusion 

 of two or more individuals, such fusion being known as con- 

 jugation. 



In most cases reproduction by spores is the consequence of 

 conjugation, but in the Infusoria etc. where the fusion at conju- 

 gation is temporary (except Vorticella), there is probably merely 

 a renewed activity a rejuvenescence which most likely results 

 in active fission or budding. In the Gregarinidae reproduction 

 by spores usually follows conjugation, but may also take place 

 without it. In some Flagellata reproduction by spores follows 

 the conjugation of two individuals in a different stage of de- 



