2 3 o COMPARATIVE ANATOMY 



pronucleus. Immediately after separation the original mega- 

 nucleus undergoes changeswhich 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 meganuclei 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 of 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 meganuclei 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 meganuclei 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 mega- 

 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 meganuclei 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 mega- 

 nucleus and a single micronucleus, both derived from the 

 combination nucleus. These Paramecia feed and multiply by 

 transverse division at the normal rate of two or three divisions 

 in the twenty-four hours. 



The course of conjugation is in all essentials the same in the 

 other members of the genus Paramecium as in P. caudatum, 

 but the details are slightly different. In P. bursaria the final 

 stages are somewhat simplified, and resemble those of Colpi- 

 dium colpoda, a ciliate infusorian whose conjugation is repre- 

 sented in the diagram (fig. 49) for comparison. It will be 

 observed that the behaviour of the micronucleus up to stage 

 G 1 is identical with that of P. caudatum, but that four daughter- 

 nuclei are formed from each combination nucleus instead of 



