266 



THE CELL 



in one very interesting peculiarity, namely, that their nuclear 

 apparatus has split up into two kinds of nuclei, which differ 

 physiologically, i.e. into the chief nucleus (macro-nucleus) (Fig. 

 146 k), and into one or more sub-nuclei or sexual nuclei (w, k) 

 (micro-nuclei). If plenty of nourishment be present, the Infu- 

 soria, which may be cultivated for observation in a small drop of 

 water, multiply by means of the usual transverse division (Fig. 



Fig. 146. 



Fig. 147. 



Fig. 146. Paramcecium caudat>xm (semi-diagrammatic) (R. Hertwig, Zool., Fig. 139) : 

 k nucleus ; nk paranucleus ; o mouth aperture (cytostom) ; na' food vacuole during process 

 of formation; na food vacuole ; cv contractile vacuole in contracted condition ; cv' contrac- 

 tile vacuole in extended condition ; t trichocysts; t' the same extended. 



Fig. 147. Paramecium aurelia, undergoing process of division. Fig. 2 shows how at 

 an earlier stage the cytostom of the lower animal is formed by means of constriction from 

 the upper one (R. Hertwig, Zool., Fig. 140) : k, nk, o, nucleus, paranucleus, and mouth 

 aperture of upper portion ; , nk', o, nucleus, paranucleus, and mouth aperture of lower 

 portion. 



147), when the macro- and micro-nuclei extend themselves simul- 

 taneously in a longitudinal direction and divide. 



This asexual multiplication is so energetic under favourable 

 conditions that a single individual may, during the period of six 

 days, divide thirteen times, and thus produce about 7,000 or 8,000 

 descendants. 



However, it has been shown, especially by the culture experi- 

 ments of Maupas and Richard Hertwig, that an Infusorian is un- 

 able to maintain the species for any length of time, and to continue 

 to multiply by simple division, even if nourishment be supplied to 

 it. The individuals undergo changes with regard to the nuclear 

 apparatus ; they may even completely lose it, when they no longer 



