194 



PROTOZOA 



The periods of fission are interrupted from time to time by the sexual 

 process of conjugation, which will be described as it occurs in Paramcecium 

 (fig. 151). Two individuals touch by their whole ventral surfaces, so 

 that their cytostomes come together. In the neighborhood of the latter a 

 bridge of protoplasm connects the two animals. Later the individuals 

 separate. While these easily observable external processes are occurring 

 there is a complete modification of the nuclear apparatus in the interior. 

 The macronucleus increases in size, and breaks into small portions which 

 disappear within the first week after copulation (probably by absorption), 

 and give place to a new nucleus derived from the micronucleus. At the 

 beginning of copulation the micronucleus becomes spindle-shaped, 

 divides .and repeats the process, the result being the formation of four 

 spindles in each animal, three of which break down, thus recalling the 

 polar globules in the maturation of the egg (p. 133). The fourth or 

 principal spindle places itself in the neighborhood of the cytostome at 

 right angles to the surface and divides into two nuclei, the superficial 

 being called the wandering or male nucleus, the deeper, the stationary or 



female nucleus. The male nuclei 

 of the two copulating animals are 

 exchanged, traversing the proto- 

 plasmic bridge in their course (III). 

 Both male and female nuclei usually 

 become spindle-shaped, and the im- 

 migrant male spindle fuses with the 

 female spindle, forming a single 

 spindle of division. At last, after 

 processes which differ in the various 

 genera, the division spindle pro- 

 duces (usually by indirect means) 

 two nuclei, one of which becomes 

 the new macronucleus, the other 

 the new micronucleus. 



In a comparison of the fertiliza- 

 tion of the Metazoa, the female 

 nucleus corresponds to the egg 

 nucleus, the male nucleus to that of the spermatozoa. As the fusion 

 of egg and sperm nuclei forms a segmentation nucleus, so here the divi- 

 sion nucleus is formed in a similar manner. As the egg cell through 

 fertilization acquires the capacity not only to produce sex cells but 

 somatic cells cells which carry on the common functions of the body 

 the fertilized micronucleus forms not only the new micronucleus, but 



FIG. 152. Epistylis umbellaria (after 

 Greeff). Part of a colony in 'bud-like' 

 conjugation; r, microspores arising by 

 division; k, microspore conjugating with 

 a macrospore. 



