208 PROTOZOA. 



ing the protoplasmic bridge in their course. Both male and 

 female nuclei become spindle-shaped, and the immigrant male 

 spindle fuses with the female spindle, forming a single spindle of 

 division. At last the division spindle produces (usually by indi- 

 rect means) two nuclei, one of which becomes the new macronu- 

 cleus, the other the new micronucleus. 



In a comparison of the fertilization 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 division nucleus is formed iu 

 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, Vjut also the 

 macronucleus which controls the body j^rocesses, and hence is the 

 somatic nucleus. In other words, fertilization in the Ciliates 

 leads to a comf)lote new formation of the niTcleus and thus to a 

 new organization of the organism. 



In most Ciliata the conjugating individuals are equivalent, 

 the fertilization is mutual, and the individuals separate later. In. 

 the Peritricha (mostly sessile forms, fig. 147), on the contrary, the 



<■ 





Fig. U7. —Episli/lis umbelhn-in, (After Greeft.t Part of a colony in 'bud-like ' conju- 

 gation ?-, microsporea arising by division; /», microspore conjugating with a 

 macrospore. 



resemblance to fertilization in the Metazoa is strengthened in that 

 there is a sexual differentiation and a permanent fusion of the 

 conjugating individuals. Some animals — the macrospores — retain 

 their size and sessile habits; others by rapid division produce 



