206 



PROTOZOA. 



The nuclear relations are extremely interesting in that there are 

 two nuclei physiologically unlike. The larger of these (nucleus 

 of older writers, macronucleus} is a large oval, rod-like, or spiral 

 body, readily and deeply staining with microscopic stains, and sur- 

 rounded with a membrane. It appears to control all the common 



vital functions of the animal (motion, 

 feeding, etc.). Beside it or in a depres- 

 sion in it is the much smaller micro- 

 nucleus (nucleolus or paranucleus of 

 older authors) which stains less deeply 

 and only plays a part in reproduction. 

 In all sexual processes it comes to the 

 front and can be called the sexual 

 nucleus. 



Multiplication of Ciliata occurs by 

 binary fission (fig. 145); more rarely, and 

 then only in the encysted condition, by 

 division into numerous (up to 64) parts. 

 Budding is known in the Peritricha and 

 Suctoria. First the micronucleus divides 



no. M-ParamKcium aurelia mitoticall y> and the * th macronucleus 



in division, ft, macronucleus; separates bv elongation and construction. 



n/r, micronuclei; o, cytostome J 



of the separating individuals. The old cytostome persists in the anterior 



At 2 an early stage of division 



of cytostome. offspring, but often an outgrowth from 



it (fig. 145, 2, o f ) passes into the posterior half and develops into 

 a new mouth. 



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. 146). Two individuals touch at first in front, 

 and then by their whole ventral surfaces, so that their cytostomes 

 come together. In the neighborhood of the latter a plasma bridge 

 connects the two. Later the individuals separate. While these 

 easily observable external processes are occurring there is a com- 

 plete 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 

 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, three of which break down, 

 thus recalling the polar globules in the maturation of the egg 

 (p. 146). The fourth or primary spindle places itself in the neigh- 



