374 DEPTHS OF THE OCEAN 



taken place, but also upon how many have perished or been 

 carried away ; and the causes bringing about diminution, which 

 we may perhaps term factors of loss, may vary without being 

 in any way directly connected with the conditions of existence 

 of the plankton. There is one genus, at any rate, whose rate 

 of augmentation can be approximately determined. The 

 species of Ceratmin only divide their cells at night, so that 

 if we make our investigations early in the morning we can tell 

 which cells have been divided during the night and which 

 remain entire. In a sample of surface-water on loth September 

 1907 we found 300 whole cells and 161 half cells of CeratitLvi 

 tripos, the latter consisting of 79 anterior parts and 82 posterior 

 parts. The number of cells, then, had in twenty-four hours 



increased from 300 -I = 380.5 on 9th September to 



300 -|- 161 =461 on loth September. The addition is accordingly 



= 80.5 individuals, and the percentage of the total amount 



10 1 1 100 X 80.5 

 on 9tn September works out at — ^ ^ = 21.2 per cent. 



This was the plan we adopted for calculating the augmenta- 

 tion of the species of Ceratimn at Drobak during the whole 

 of their vegetation period in 1907, and we also recorded the 

 average number per litre at different depths during the whole 

 year.^ The following tables show our chief results : — 



^ Similar investigations in the case of Ceratiuni tripos were carefully carried out during 

 1908-1909 by Apstein in the Baltic. The values he obtained for percentages of augmentation 

 on the whole accord as nearly with mine as might be expected. 



[Table 



