NUCLEOPLASMIC RELATIONS IN ARCELLA 8/ 



two species is not begun until there is a sufficient number of 

 nuclei in the parent to fully supply both parent and offspring. 



Empty shells were not formed when changes from a greater 

 to a lesser number of nuclei occurred, because there were nuclei 

 enough for both parent and offspring, and hence division of 

 the cytoplasm could proceed as usual. 



Changes from the binucleate to the uninucleate condition 

 in A. dentata were probably due to the failure of the nuclei 

 to divide at the time of fission, hence both parent and offspring 

 received but a simple nucleus. 



The change from the binucleate to the trinucleate condition 

 noted in A. vulgaris may have resulted from two successive 

 divisions of one of the nuclei of the original parent before fission 

 occurred, and the subsequent distribution of three nuclei each 

 to both parent and offspring. 



The change from a lesser to a greater number of nuclei prob- 

 ably depends upon several factors. The results of the micro- 

 dissection experiments indicate that nuclear doubling in A. 

 dentata occurs more quickly when a larger mass of cytoplasm 

 is present than normally comes under the sphere of influence 

 of one nucleus. The amount of cytoplasm is in turn 

 determined by the capacity of the shell. It has been shown 

 that variations in the dimensions of the new shell are never 

 very great, apparently being limited by the size of the parent 

 shell, and that a large internal change, such as nuclear dou- 

 bling, is not immediately followed by a correspondingly large 

 variation in the progeny, but requires several generations for 

 its complete realization so far as external characteristics are 

 concerned. It appears from these data that the uninucleate 

 specimens remained uninucleate until a variation in the dimen- 

 sions of the shell occurred which increased the shell capacity 

 sufficiently to allow an increase in the cytoplasm beyond the 

 amount usually associated with a single nucleus. The presence 

 of cytoplasm outside of the 'sphere of influence' of a single nucleus 

 may be the stimulus that initiates the process of nuclear dou- 

 bling. Thus the generation in which this process occurs is not 

 definitely fixed, but depends upon variations in the shell capacity 

 of the organism. 



