GERMAN PLANKTON INVESTIGATIONS. 295. 



catch, diluted with water until a given volume is 

 reached. This is the first dilution. If the catch has been 

 preserved in alcohol, the latter must be thoroughly washed 

 out with water, and this operation takes several days. 

 Suppose the volume of the catch has been diluted with 

 water to 50 ccm., it is evident that in each ccm. of this 

 dilution the different organisms are present to a very 

 varying extent. When the volume has been thoroughly 

 shaken up, we find perhaps in one ccm. one Leptodora and 

 300,000 Melosira. The examples given in the estimation 

 detailed below are from fresh water plankton. An even 

 greater variation occurs in the case of salt water 

 organisms. It may here be mentioned that the method 

 of enumeration and the forms employed for entering the 

 results of the same are exactly the same for both salt and 

 fresh water planktonic investigations. Now the enumera- 

 tion of 300,000 Melosira is obviously impossible, so for 

 the enumeration of the more abundant individuals we have 

 to make a second dilution. In the present instance we 

 take from the 50 ccm. of the first volume, say 2"5 ccm., 

 after the volume has been well shaken up in order that 

 the organisms may become as evenly distributed as pos- 

 sible, and dilute this a second time until it becomes 

 50 ccm., then we have in this second dilution in every ccm. 



300,000 1C ftnn ', . _ ,,. , ,., ,. 



= 15,000 Melosira. Irom this second dilution 



50x2-5 



we count out the number of Melosira in one-tenth of a 



ccm., that is, 1,500. In this mass of water it is possible 

 that we should not find any individuals of the rarer 

 species, so that when we wished to count these, the dilu- 

 tion would not have to be carried so far, that is, 10 ccm. 

 of the first dilution would be diluted up to 50 ccm. For 

 the still rarer forms the first volume itself would have to 

 be taken for the purpose of counting. 



