16 CELL ORGANS [CH. 



constant conditions there does seem to be a normal ratio for 

 each species, and causes which seriously upset the ratio lead 

 to the death of the animal. For example, in forms such as 

 Difeptus, in which the nucleus is fragmented, or in Actino- 

 sphaerium, in which there are many small nuclei, when the 

 organism is subjected to prolonged starvation the cyto- 

 plasm becomes reduced, but the karyo-plasmatic ratio is 

 preserved by the disappearance of some of the nuclei or 

 nuclear fragments, and such organisms can survive very 

 prolonged starvation. But in Paramoecium, in which there 

 is one large nucleus, when the animal is starved the cyto- 

 plasm shrinks but the nucleus seems unable to undergo a 

 corresponding diminution, and in such forms starvation 

 rapidly results in death. 



HERTWIG regards a disturbance of the normal karyo- 

 plasmatic ratio as the immediate cause of cell-division, and 

 supposes that the unequal rate of growth of nucleus and 

 cytoplasm brings about a condition of "karyo-plasmatic 

 strain " leading to cell-division and a consequent restoration 

 of the normal ratio. This process has been investigated in 

 detail by POPOFF (1908-9), who finds that the period of 

 strain occurs some time before division, since the ratio im- 

 mediately before and immediately after division is the same. 

 In certain Ciliate Protozoa, for example, after division the 

 cytoplasm increases in volume more quickly than the 

 nucleus until a condition of strain is produced, after which 

 the nucleus increases very rapidly until another division 

 occurs. If, before the strained condition is reached, a 

 portion of the cytoplasm is cut off, division is delayed until 

 the lost portion is restored by further growth; but if the 

 condition of strain has been reached before the piece is cut 

 off, the necessary stimulus to division has been given, and 

 the organism divides in spite of the fact that the excess of 



