22 BRITISH FRESHWATER RH1ZOPODA. 



REPRODUCTION. 



Several modes of reproduction have been observed 

 among the Rhizopoda, but, from the infrequency of 

 their occurrence, some have not been satisfactorily 

 worked out. The modes most frequently noticed are 

 (1) simple fission or binary self-division; and (2) spore- 

 formation. The latter is little more than the breaking 

 up of the plasma into fragments, and the development 

 of these ; each separate portion being the germ, or 

 earliest stage in the existence, of a new individual. 

 This process is usually preceded by the encistment of 

 the adult individual ; that is to say, the formation, upon 

 the withdrawal of the plasma into the interior of the 

 test (in the Conchulina) of a spherical or oval cist, 

 which acquires a hard (chitinous or possibly siliceous) 

 coat. The cist remains quiescent for a longer or 

 shorter period, until division of the plasma and nucleus 

 takes place, and the " spores " are liberated. Fre- 

 quently but, according to Ray Lankester, not neces- 

 sarily two (rarely three or more) individuals come 

 together and fuse before breaking up into spores. 

 This process is known as "conjugation"; and there 

 can be no doubt, says the same authority, that the 

 physiological significance of the process is similar to 

 that of sexual fertilization, namely, that the new spores 

 are not merely fragments of an old individual, but are 

 something totally new, as they consist of the substance 

 of individuals which previously had different life- 

 experiences. Whilst spore-formation is not necessarily 

 preceded by conjugation, conjugation is not necessarily 

 followed by spore-formation. Professor Lankester 

 further remarks : " There is certainly no marked line 

 to be drawn between reproduction by simple fission, 

 and reproduction by spore-formation ; both are a more 

 or less complete dividing of the parent protoplasm into 

 separate masses ; whether the products of the first 

 fission are allowed to nourish themselves and grow 



