REPRODUCTION 225 



From this brief survey it is quite evident that far-reaching changes 

 of the protoplasmic organization take place at periods of division. 

 Both nuclei and cytoplasm are necessary but the micronucleus 

 apparently may be lost without destroying the power of the cell to 

 divide. Amicronucleate races of ciliates, arising possibly through 

 defective reorganization and division after conjugation (see Moore, 

 1924), have been maintained in culture for many generations by 

 division, although they are ultimately lost (see (Chapter VII). On 

 the other hand, the power to regenerate is connected in some manner 

 with the micronucleus. Thus young cells of Uronychia transfuga, 

 when transected with a scalpel, will regenerate only that fragment 

 which contains the micronucleus (Calkins, 1911, Fig. 113; Young, 

 1923). In old cells, however, both fragments regenerate regardless 

 of the presence or absence of a micronucleus, a fact indicating a 

 change in organization with advancing age (Fig. 113, 5). 



The fate of the motorium and of the coordinating fibrils both 

 endoplasmic and those of the silver line system, at division is still 

 unknown. It is a significant fact that the peristome and the peri- 

 stomial organs appear first in the more specialized anterior half of 

 the ciliate cell, and from this position gradually shift to the region 

 immediately posterior to the division zone (Figs. 109, 110). The 

 relation of the posterior mouth to the silver line system in a dividing 

 form of Glaucoma scintillans is clearly shown by Chatton, Lwoff 

 (A. and M.) and Monod (1931). The complicated oral membranes 

 of this organism are formed as a result of division of the blepharo- 

 plasts at a localized region of certain lines of the silver line system 

 (Fig. 114). In Vorticella according to Biitschli (1888) after Fabre, 

 the peristome and adoral zones are reversed in the daughter cells. 



II. UNEQUAL DIVISION (BUDDING OR GEMMATION). 



In reproduction by budding or gemmation, one or more minute 

 fragments of the cell are produced by unequal division of the 

 organism. Parent and offspring are thus distinguished, their rela- 

 tive sizes varying in different cases. In many instances both parent 

 and offspring continue to live after such reproduction. In many 

 other instances the residual parental protoplasm is no longer able to 

 carry on metabolic activities and dies. Illustrations of both types 

 abound in all groups of the Protozoa, the buds being formed either 

 on the periphery of the parent in so-called exogenous budding, or 

 within the protoplasm of the parent in so-called endogenous budding. 

 The minute cells that are formed by budding always contain a por- 

 tion, sometimes one-half, of the nuclear structures of the parent 

 and may develop asexually into organisms similar to the parent, or 

 they may be differentiated as gametes requiring fertilization before 

 development. 

 15 



