G EMBRYOLOGY. 



to and apparently in consequence of a very important process known 

 as conjugation, which consists in the temporary or permanent fusion 

 of two or more individuals, or spontaneously, i.e. independently of any 

 such previous conjugation. 



Reproduction by fission consists simply in the division of the 

 organism into two similar parts, the nucleus when present becoming 

 divided simultaneously with the cell body. This mode of reproduction 

 is the simplest conceivable, and is not followed by a development, 

 since the two organisms produced are exactly similar, except in 

 size, to the parent form. Besides single fission, a process of multiple 

 fission may take place, as amongst the Flagellata, where Drysdale 

 and Dallinger have shewn that an individual enclosed within a 

 structureless cyst may divide first into two, then into four, and so on. 



The process of budding differs mainly from that of simple fission 

 in the fact that the two organisms produced are dissimilar in size, and 

 also that the separation of the smaller organism from the larger is 

 preceded by a process of growth in the latter, so that in the separation 

 of the bud no essential part of the parent form is removed. This mode 

 of reproduction is found amongst the Infusoria, Acineta, &c. An in- 

 teresting variation in it is the internal gemmation of many of the 

 Acineta, where a portion of the internal protoplasm with part of the 

 nucleus is separated off to form a fresh individual. This mode of 

 gemmation is connected by a series of gradations with the normal 

 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 forma- 

 tion of colonies. 



The third mode of reproduction, by spore formation, does not essen- 

 tially 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 distin- 

 guished 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 flagellurn, etc. They may even be with- 

 out a nucleus when the parent organism is nucleated, as in the 

 Gregarinidye. 



The encystrnent, 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 reproduction. 



As has been already stated, all the above modes of reproduction 

 take place in some of the Protozoa without any anterior process 



