78 



GENERAL EMBRYOLOGY 



cytoplasm, i.e., (y^) . In many cells during, and immediately after, 

 division the nucleus grows at the expense of the cytoplasm, and the 

 ratio (^) is raised (Fig. 40). The cell then enters upon its vegetative 

 phase, during which the cytoplasm grows more rapidly than the nucleus, 

 and the ratio diminishes toward the lower functional limit; as the ratio 

 approaches this limit the functional activities of the cell change, and the 

 normal vegetative processes give place to that form of action which we 

 call cell division, during which the ratio again rises to a value permitting 

 normal vegetative functioning. It is not yet possible to state in 

 precise quantitative terms what the limits of these ratios of volume 



2 



1.8 



1 b 



C 



1.9 



1,8 



1.4 



16 17 



FIG. 40. Curve showing the increase in volume of nucleus (a-d) and cytoplasm 

 (6-c) during inter kinesis, in the Infusorian; Frontonia leucas, at temperature of 

 25 C. After Popoff. Ordinates, volume; abcissas, time in hours. Each curve 

 shows a doubling of volume (1:2) during the seventeen hour period of the inter- 

 kinesis. Each curve is based upon its own units of measurement, which are 

 different for the two curves. The nucleo-cytoplasmic relation is identical at the 

 beginning and end of the period. The size of the nucleus is relatively smallest at 

 fifteen hours; then the nucleus begins to grow very rapidly, so that at the time 

 of the division of the whole cell, the original relation is restored. 



and surface are in specific instances, nor even to say whether the volume 

 or surface, or volume and surface relations are those essentially involved. 

 But so far this nucleo-cytoplasmic hypothesis is the most plausible ex- 

 planation of the nature of the immediate conditions of cell division. 

 It should be said, however, that the applicability of Hertwig's " Kern- 

 plasma Relation" is still chiefly limited to Protozoan cells, and that 

 even here there are many contradictions. Some of the more obvious 

 exceptions to the definition of such a limiting ratio are to be ex- 

 plained as special adaptations. Such are the very great cytoplas- 

 mic bulk of many egg colls or the relatively large size of the nucleus 

 in the sperm cell. Many other exceptions of special character are to 



