INTRODUCTION. 



velopment. Thus in the springing Monad, described by 

 Drysdale and Dallinger, a form produced by the fission of a 

 monad in an amoeboid condition fuses with an ordinary monad 

 to produce an individual, which then breaks up into spores. 

 Another instance of the fusion of dissimilar individuals is 

 afforded by Vorticella, where a free-swimming individual conju- 

 gates and is permanently united with a fixed one (Engelmann, 

 Butschli). Conjugation often consists in the fusion of more than 

 two individuals. In conjugation where the fusion is permanent, 

 the nuclei of the conjugating forms usually unite before the 

 product breaks up into spores ; and where temporary fusion 

 occurs in the Infusoria a division of the paranuclei and often of 

 the nuclei takes place, followed by the ejection of parts of them, 

 and a reproduction of new paranuclei and nuclei from the 

 remainder of the original structures. 



In order to understand the meaning of conjugation in con- 

 nection with reproduction, it is important to understand how the 

 two became in the first instance related. For the solution of 

 this question the fact that many Protozoa have the capacity of 

 temporarily or permanently fusing together without an imme- 

 diate act of reproduction is of great importance. A good example 

 of such fusion is supplied by Actinophrys. We must suppose in 

 fact that the simple coalescence of two or more individuals gives 

 a sufficient amount of extra vigour to their product, to compen- 

 sate the race for the loss in number of individuals so caused. 

 This extra vigour probably first exhibited itself especially by 

 increased activity in reproduction, till finally the two processes, 

 viz. that of conjugation and that of reproduction, came to be 

 inseparably connected together. 



The reproduction of the forms above the Protozoa, which are 

 known as the Metazoa, takes place by two methods, viz. a sexual 

 and an asexual one. The sexual process, which occurs in every 

 known Metazoon 1 , consists essentially, as is shewn in the second 

 chapter of this work, in the fusion of two cells budded off from 

 the parent organism, viz. the female cell or ovum, and the male 

 cell or spermatozoon, and of the subsequent division of the 

 compound cell so produced into a number of parts which build 



1 Dicyema, if it is a true Metazoon, would seem to form an exception to this rule. 



