INTRODUCTION. 1 



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 conjugation. 



In most cases reproduction by spores is the consequence of 

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

 is temporary (except Vorticella), there is probably merely a renewed 

 activity a rejuvenescence which most likely results in active 

 fission or buckling. In the Gregarinidce 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 development. Thus in the 

 springing Monad, described by Drysdale and Dallinger, a form 

 produced by the fission of a monad in an amcBboid 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-swimrning individual conjugates 

 and is permanently united with a fixed one (Engelmann, Biitschli). 

 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 connection 

 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 immediate act of reproduction is of great 

 importance. A good example of such fusion is supplied by Actino- 

 phrys. 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 compensate 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, 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 them- 

 selves up into an organism resembling one of the parents. The 

 sexual process has obviously at first sight a very close resemblance to 



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



