11)4 



PROTOZOA 



The periods of tission aie interrupted from lime to time l>v the sexual 

 process of conjugation, whicii will be descrilied as it occurs in Para Dim in in 

 (tig. 151). Two indivitluals touch l)y their whole A-entral surfaces, so 

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

 bridge of ]>rotoplasm connects tire two animals. Later the individuals 

 separate. \\'hile these easily obser\a,ble external processes are occurring 

 there is a complete modilicalion of the nuclear a|)paratus in the interior. 

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

 disappear witliin the Ih-st week after copulation (probably by absorption), 

 and give place to a new irucleus derived from the niicronucleus. At the 

 beginning of copulation the nricromicleus becomes s])indle-sha])ed, 

 divides and repeats the process, the result being the formatioir of four 

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

 polar globules in the maturation of the egg (p. 1,13). 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 superhcial 

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



female nucleus. The male nuclei 



rr^^!'^, ^O 



of the two copulating animals are 

 exchanged, tra\ersing 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 sijigle 

 spindle of division. At last, after 

 processes which differ in tire various 

 genera, the division spindle pro- 

 duces (usuall)- by indirect means) 

 two nuclei, one of wliich l)ecomes 

 the new macronucleus, the other 

 the new niicronucleus. 



In a comparison of the fertiliza- 

 tioir of the Metazoa, the female 

 nucleus corresponds to the egg 

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

 of egg and sperm nuclei forms a segmentation nucleus, so here the di\i- 

 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 Ixxly 

 — the fertilized niicronucleus forms not only the new niicronucleus, but 



Fio. 15J. — lifi'nlylh ilnihi-Uari,: (after 

 ("ircfff). Part of a coldiiy in ' Iniil-likr' 

 conjugation; /', nii(Tos|iori.-s arising liy 

 <HvIsIon; />', microspore conjugating wllii 

 a macros] >orc. 



