PROTOPLASM OF PROTOZOA 45 



undergo certain sexual phenomena, or to form cysts. However, environ- 

 mental conditions, such as amount and kind of food present, tempera- 

 ture, and so forth, have been generally observed to affect the rate at 

 which these phenomena take place. Since Protozoa under normal condi- 

 tions seem to be limited to a more or less uniform maximum size for 

 a given species, various suggestions have been made to the effect that 

 the stimulus for division is in part controlled by definite ratio of volume 

 to body surface, or to a possible ratio of nuclear volume to cytoplasmic 

 volume. However, whatever may prove to be the explanation of this, 

 it is an observed fact that in most cases conditions suitable for most 

 rapid growth are the same as those best suited for most rapid division. 

 On the other hand, relatively unfavorable conditions, such as scarcity 

 of food or the concentration of metabolites, have been observed to in- 

 duce sexual activity in the various Protozoa (see Giese, 1935, for litera- 

 ture). Practically nothing can be said of the physical changes under- 

 gone by the protoplasm of the organisms just before they start division, 

 conjugation, or endomixis, except that before division they may shorten 

 and thicken {Uroleptus, Calkins, 1919), or become spheroidal in form 

 {Amoeba, Chalkley, 1935). Such observations possibly indicate a change 

 in viscosity, the nature of which awaits investigation. 



In certain of the hypermastigote flagellates Cleveland (1934, 1935) 

 has found one of the best materials for the study of the cytology of 

 cell division, not only in the Protozoa but of cells generally. Here the 

 mitotic spindle, with its chromatic and achromatic parts, is clearly seen 

 in the living condition; it may also be easily preserved and stained by 

 ordinary techniques. In addition, experiments on pulling the centrosomes 

 demonstrate the elasticity and contractility of the extranuclear chromo- 

 some fibers and those of the central spindle. Accordingly, in this ma- 

 terial the various elements of the mitotic apparatus must be considered 

 real and therefore demand careful consideration in any discussion of the 

 mechanics of mitosis, not only in these forms but in astral types of 

 mitotic divisions generally. Furthermore, a study of the physical prob- 

 lems involved in the origin and nature of the centrosomes, spindle 

 fibers, asters, degeneration and reorganization of certain of the locomotor 

 organs, as well as the correlated mechanics of cytokinesis in these forms 

 would be most valuable. However, for the present we can only assume 

 that the physical-chemical changes giving rise to these various struc- 



