GERMAN PLANKTON INVESTIGATIONS. 311 



a large and important part of the food supply of fish. On 

 the other hand, the Diatoms, in comparison with the other 

 organisms of the plankton, contain a proportionately very 

 small amount of dry organic substance, but they exist in 

 such colossal quantities in the plankton that Hensen was 

 able to demonstrate that by far the larger quantity of the 

 organic substance of the plankton exists in the form of 

 Diatoms. 



The chemical analysis method has been further 

 developed by Brandt (7), but it is more convenient to first 

 of all consider Hensen's results deduced from the method 

 by counting. 



The most important results deduced from this method 

 are in connection with the enumeration of the floating fish 

 eggs. The first results were obtained in the West Baltic 

 for cod and flat fish. The results of 120 such observations 

 are detailed. These results are in some respects deficient, 

 because the percentage of salt in the Baltic, and therefore 

 the specific gravity of the water, varies greatly, so that 

 occasionally the specific gravity of the water fell below 

 the point at which it was possible for the eggs to float. 

 The numbers must therefore be considered as minimal. 

 Hensen concluded that for the Eckenforde waters, where 

 the fishery for cod and flat fish is carried on (an area of 

 about 16 square miles) there are in January an average 

 of 30, in February from 45 to 50, in March at least GO, and 

 in April 50, floating eggs of the above fishes per square 

 metre of surface (with an average depth of 20 metres). 

 These eggs take on the average 15 days to develop under 

 the conditions obtaining in the West Baltic, so that the 

 number above recorded must be doubled in order to give 

 the number occurring per month under a square metre of 

 surface water. This gives from January to April 370 

 eggs. 



