THE INFUSORIA 453 



is to produce small individuals containing, as a rule, from three to six secondary 

 nuclei. Such individuals become encysted (infection-cysts), and pass out of 

 the frog in the fasces. The animal at first fills the cyst completely and shows 

 no cilia, but after a time the body shrinks within the cyst, and the animal is 

 then seen to have a ciliary covering. 



The faeces of the frogs are readily devoured by tadpoles, which thus become 

 infected with cysts. In the gut of the tadpole the Opalina emerges from its 

 cyst, it at once divides up into uninucleate individuals, the gametes, 

 elongated club-shaped forms with a sparse 'coat of cilia over the flattened 

 body. Under unfavourable circumstances the gametes undergo agglomera- 

 tion in rosettes, adhering by their pointed ends (Neresheimer). Under 

 normal circumstances they copulate in pairs as isogametes, according to 

 Neresheimer, inO. ranarum; butMetcalf describes smaller uninucleate micro- 

 gametes and larger macrogametes with one or two nuclei, in other species ; 

 the male pronucleus then fuses with one of the two nuclei of the macrogamete. 

 The zygote of 0. ranarum, with the two pronuclei still separate, rounds itself 

 off and becomes encysted (copulation-cyst) ; within the cyst the two pronuclei, 

 which have passed into a spindle-stage, undergo fusion. The zygote emerges 

 from the cyst with a synkaryon, and it becomes an adult Opalina. 



Neresheimer considers that the life-cycle of Opalina proves that its affinities 

 are with Plagellata rather than with Infusoria. In deciding this question, 

 it must be considered, in the first place, whether in such a form the life-cycle, 

 or the structural features of the body, are most likely to indicate affinity 

 that is to say, least likely to exhibit secondary peculiarities due to adaptation. 

 Opalina is a parasitic form, and its life-cycle shows very obviously a direct 

 daptation, of a type very common in parasitic Protozoa, to its mode of life ; 

 multiplicative reproduction increasing its numbers within the host, and prop- 

 agative reproduction, combined with sexual phenomena, leading to the 

 infection of new hosts. On the other hand, its minute structure is that typical 

 of Ciliata, a character hardly likely to be due to the influence of parasitism, 

 as Popoff (125) has well pointed out. 



The chief difference between Opalina and other Ciliata, which requires 

 special consideration, is the fact that the animal contains but one kind of 

 nucleus. This, however, is a character known in other genera of Ciliata 

 &\so--e.g., Trachdocerta, Ichthyophthirius. There can be but little doubt 

 that the " heterokaryote " condition of the Infusoria, with distinct generative 

 and vegetative nuclei, must have been derived phylogenetically from a condi- 

 tion in which, as in other Protozoa, the two kinds of chromatin were contained 

 in one and the same nucleus ; and to find this condition still retained in some 

 Infusoria would not be remarkable. In such forms it is to be expected that 

 prior to gamete-formation the vegetative chromatin, equivalent to the 

 macronucleus, would be expelled, and the pronuclei would be formed from 

 generative chromatin. 



There is nothing, therefore, to be said against the view of Popoff, that 

 Opalina shows the most primitive type of gamete-formation known at present 

 amongst the Ciliata. Its nuclei contain generative and vegetative chromatin 

 combined, and in preparation for syngamy nuclei are formed which are purely 

 generative, out of chromidia expelled from the primary nuclei. The forma- 

 tion of uninucleate gametes which copulate, (total karyogamy) has been re- 

 garded by almost all those who aave theorized on the subject as being probably 

 the most primitive type of syngamy from which the conjugation (partial 

 karyogamy) of the Ciliata has been derived (p. 154). 



In TracJielocerca (p. -loO) the gamont produces in a similar manner a number 

 of generative nuclei (micronuclei) prior to the syngamic process ; but here, 

 as in Ciliata generally, the gumont no longer divides into a number of gametes ; 

 only one micronucleus in each gamont persists to form the two pronuclei, and 

 the usual process of partial karyogamy takes place. These considerations 

 indicate that the monomorphic character of the Infusorian life-cycle is a 

 secondary feature ; as the structural complication of the body has increased, 

 so the tendency to divide up into relatively minute swarm-spores has been 



