344 



THE AMERICAN NATURALIST [Vol. LV 



one division figure were usually older than those contain- 

 ing one only. Thus the average nuclear number of 

 specimens with one dividing nucleus was 7.2, with two 

 dividing nuclei, 8.1, with three dividing nuclei, 9.3, and 

 with 4 dividing nuclei, 10. These data favor the conclu- 

 sion that the stimulus that initiates nuclear division acts 

 as a rule on only one nucleus at a time and that the divi- 

 sion of this nucleus restores the nucleo-cytoplasmic ratio. 

 When this ratio is again disturbed by an increase of the 

 cytoplasmic mass another nucleus is stimulated to divide. 

 Division of two or more nuclei synchronously may be due 

 to the more rapid growth, the larger specimens in which 

 this usually occurs, or to the greater chances of two or 

 more nuclei reacting to the division-stimulus when a large 

 number of nuclei are present in a single specimen. There 

 is some evidence that the nucleus that undergoes division 

 is the one with the greatest amount of cytoplasm sur- 

 rounding it, but this could not be determined definitely. 

 No regular distribution of the nuclei was evident. It is 

 interesting to note in this connection that during the em- 

 bryonic development of many animals nuclear division 

 occurs in all cells at nearly the same time. This is espe- 

 cially interesting in the case of certain insects, in the eggs 

 of which nuclear division proceeds sj'nchronously without 

 the intervention of cell walls until thousands of nuclei 

 are present in a single egg.^ An increase of cytoplasm 

 over nucleus may also, in these insect eggs, stimulate 

 nuclear division, since after each division the mass of 

 cytoplasm surrounding each nucleus is increased by the 

 addition of new material elaborated from the yolk sub- 

 stance in which it is situated. 



8. The average total volume of the nuclei of certain 

 specimens, average volume per nucleus, average total 

 area of the surface of tlu> nuclei niid average area of the 

 surface per nucleus wet-e iiie.-isin-ed in cuhie niilliiiielers 

 and square millimeters tn.m (•;iim i;i dr.-iwi ims wliieli 



*Hegner, E. W., 1914, "Stmlvs un <;. r,M C.ll-," ,/,.-/,-. 2r,: :;75- 



509. Especially pp. 408-41.!. 



