SEX IN INFUSOEIA 225 



of fission the micronucleus retains its small dimensions ; and 

 freshly separated daughter microniiclei are always connected 

 by a long fibrous strand, whose terminal swellings (i. e. groups 

 of chromosomes) stain deeply with all nuclear stains. In 

 cases of progamic fission (PI. 17, fig. 7) the posterior individual 

 does not show any signs of a skeletal plate from the beginning 

 to the end of the fission — it is the microconjugant in statu 

 nascendi, and gets its long and slender terminal spine from 

 its neuter parent. The anterior individual, which gets the 

 skeletal plate of its parent, differs from the latter by the 

 shortness and thickness of its newly developed terminal spine. 



The progamic fission can be easily recognized, moreover, 

 by the behaviour of the micronucleus, which swells to enormous 

 size, becoming at the same time very feebly stainable with 

 nuclear stains (PI. 17, fig. 7, Mi). The micronuclei of both 

 the daughter individuals remain in this condition from the 

 moment of their disjunction to the beginning of conjugation. 

 Such individuals may be termed preconjugants (PI. 17, figs. 8 

 and 9). From what has just been said it follows that in a 

 population of 0. janus one can easily distinguish the 

 preconjugants from other individuals (neuters) which are 

 incapable of conjugating. 



The conjugation of . janus becomes still more interesting 

 from the fact that this species exhibits a certain percentage 

 of isogamous pairs, exclusively of the macroconjugant type. 

 The number of such isogamous pairs amounted, in both the 

 antelopes examined, to about 20 per cent, of the whole number 

 of pairs, the remaining 80 per cent, being of the anisogamous 

 type. On examining such isogamous pairs more closely, it is 

 seen that about 60 per cent, of them present different stages 

 of nuclear changes characteristic of conjugation (PI. 17, fig. 10) 

 from the first maturation spindle to the stage of migrating 

 pronuclei. In the remaining 40 per cent, of isogamous pairs 

 the micronuclei remain very small and lie embedded in a flat 

 depression of the macronucleus (Text-fig. 1) without showing 

 any preparation to fission. As a further peculiarity, the con- 

 jugants of such pairs appear to assume a position somewhat 



