CONJUGATION IN PARAMECIUM 199 



and then a transference of micronuclear material is effected, 

 the male pronucleus of one gamete passing over and fusing with 

 the female pronucleus of the other, and vice versa. The 

 all-important act of conjugation, the transference of nuclear 

 material has now been effected, and the gametes, which 

 have diminished in size during conjugation, shortly afterwards 

 separate from one another, begin to feed again and recover 

 their original dimensions. Each possesses the original 

 unaltered macronucleus and a rather large micronucleus, now 

 called the Combination nucleus, formed by the union of the 

 male pronucleus of the other gamete with its own female 

 pronucleus. Immediately after separation the original macro- 

 nucleus undergoes changes which lead to its final disappearance. 

 It becomes deformed by the appearance of numerous folds 

 on its surface, and at a later stage splits up into a number 

 of irregular shreds which elongate to form long interlacing 

 ribbons. Eventually these ribbons divide into a number of 

 spherical corpuscles, which are either cast out of the body or 

 incorporated with the new macronuclei subsequently formed, 

 according as the progeny of the ex-gamete are well fed or 

 starved. Meanwhile the combination nucleus in each ex-gamete 

 has undergone three successive mitotic divisions (stages F, G 1 

 G 2 in the diagram) and there are eight products in the com- 

 bination nucleus arranged in two groups of four, one group at 

 the anterior and another at the posterior end of the body. The 

 four nuclei of the anterior group increase in size and eventually 

 become the macronuclei of the progeny of the ex-gamete. Of 

 the posterior group three members atrophy and disappear, one 

 alone surviving as a micronucleus. Some twenty-four to thirty 

 hours after separation the ex-gamete divides in such a manner 

 that two of the new macronuclei pass into one product of 

 division and two into the other, whilst the micronucleus 

 (which at this stage is not very much smaller than the macro- 

 nuclei) divides mitotically, one of its products entering each of 

 the two daughter Paramecia. A second division quickly follows 

 accompanied, as before, by mitotic division of the micronucleus, 

 whilst the macronuclei are again passively distributed among 

 the products of division. The end result is that each ex-gamete 

 has produced four normal Paramecia containing each a macro- 

 nucleus and a single micronucleus, both derived from the 

 ' combination nucleus. These Paramecia feed and multiply by 



