NUCLEO-CYTOPLASMIC RELATION OF OXYTRICHA 19 



VI. CONCLUSIONS 



The results presented are suggestive from several points of 

 view. The recent extensive and thorough work of Jennings on 

 Paramaecium shows clearly that a wild population of either of 

 the common species, aurelia or caudatum, can be resolved into 

 several diverse races or genotypes which are distinguished by 

 differences in size, and that these differences are independent of 

 environmental factors. In other words, the various races main- 

 tain their relative sizes when bred under diverse conditions, pro- 

 vided only that the conditions are identical for each race. The 

 present data bear directly on this problem in that they indicate 

 the wide range of variation in size which may occur at the same 

 and at different periods of the life of a pure line of Oxytricha, and 

 emphasize the fact that even extreme variations in size are of 

 little or no specific value, and that very large numbers of cells 

 of a pure line under identical culture conditions and at the same 

 division rate must be measured in order to establish morpholog- 

 ical criteria of infusorian genotypes. In diverse races, the divis- 

 ion rate of which is different under identical environmental con- 

 ditions, the investigation is obviously still more difficult. 



Although most studies on pedigreed races of Protozoa indi- 

 cate that cell size decreases as the rate of division decreases, it 

 is usually held, from studies in embryology, that cells which di- 

 vide infrequently are larger, all things being equal, than those 

 which divide more often. For example, Conklin has shown that 

 the 'turret' cells of Crepidula are the smallest cells in the entire 

 embryo when they are formed but since they divide only twice 

 during the whole cleavage process they grow very large, whereas 

 the cells from which they are derived, the apical cells, give rise 

 during the same period to twelve cells whose combined volume 

 is not much greater than the volume of one full-grown 'turret' 

 cell. This is in accord with the present observations on Oxy- 

 tricha and those of Gregory on Tillina magna. 



It seems clear from my data that neither the size of the cell 

 nor the size of the nucleus is the inciting cause of division, but 

 rather that the rate of cell division is probably indirectly re- 

 sponsible for the size of the cell and the nucleus. The wide varia- 



