CONJUGATION IN UNICELLULAR FORMS 



I6 7 



B 



pronucleus (Fig. 82, C-H). Each animal now contains a cleavage- 

 nucleus equally derived from both the conjugating animals, and the 

 latter soon separate. The cleavage-nucleus in each divides three 

 times successively, and of the eight resulting bodies four become 

 macronuclei and four micronuclei (Fig. 82, H-K). By two suc- 

 ceeding fissions the four macronuclei are then distributed, one to each 

 of the four resulting individuals. In some other species the micro- 

 nuclei are equally dis- 

 tributed in like man- 

 ner, but in P. cauda- 

 tum the process is 

 more complicated, 

 since three of them 

 degenerate, and the 

 fourth divides twice 

 to produce four new 

 micronuclei. In 

 either case at the 

 close of the process 

 each of the conju- 

 gating individuals has 

 given rise to four 

 descendants, each 

 containing a macro- 

 nucleus and micro- 

 nucleus derived from 

 the cleavage-nucleus. 

 From this time for- 

 ward fission follows 

 fission in the usual 

 manner, both nuclei 

 dividing at each fis- 

 sion, until, after many 

 generations, conjuga- 

 tion recurs. 



Essentially similar facts have been observed by Richard Hertwig 

 and Maupas in a large number of forms. In cases of permanent 

 conjugation, as in Vorticella, where a smaller microgamete unites with 

 a larger macrogamete, the process is essentially the same, though the 

 details are still more complex. Here the germ-nucleus derived from 

 each gamete is in the macrogamete one-fourth and in the microgam- 

 ete one-eighth of the original micronucleus (Fig. 83). Each germ- 

 nucleus divides into two, as usual, but one of the products of each 

 degenerates, and the two remaining pronuclei conjugate to form a 

 cleavage-nucleus. 



D 



Fi S- 8 3- Conjugation of Vorticellids. [MAUPAS.] 

 A. Attachment of the small free-swimming microgamete to 

 the large fixed macrogamete; micronucleus dividing in each 

 (Carchesium}. B. Microgamete containing eight micronuclei; 

 macrogamete four ( Vorticell-d) . C. All but one of the micro- 

 nuclei have degenerated as polar bodies or " corpuscules de 

 rebut." D. Each of the micronuclei of the last stage has divided 

 into two to form the germ-nuclei ; two of these, one from each 

 gamete, have conjugated to form the cleavage-nucleus seen at 

 the left ; the other two, at the right, are degenerating. 



