324 Vfiriatioit and Correlation in Arcella 



The Arcellae were then taken with a pipette from tho culture dish aiul placed 

 a few at a time ou a slide in water, and covered with a cover-slip. Then with 

 a sharp needle the points niarking the limits of the maximum diameter of the 

 shell and of the " mouth " opening as seen in the caniera iniage were pricked on 

 a Card of smooth drawing-paper laid on tho table under the inirror of the caniera. 

 After the size of all the Arcellae on a slide had been pricked in this way, the slide 

 was cleaned off and a fresh lot of imlividiKils taken from the culture. 



In this way cards were obtaincd bearing the dimensions magnified appro.xi- 

 mately 400 times, and raarked by needle prick.s. These dimensions ou the cards 

 were then measured by uieans of a sliding arm calliper, bearing a vernier reading 

 to fiftieths of a millimeter. The mcasurements so obtained were divided by the 

 magnificatiou of the microscope-camcra combination and reduced to mikrons. The 

 measurements in mikrons were then permanently i-ecorded. 



From a series of test measurements (50 in number) of distances of known size 

 (spaces of a net micrometer) niade by this method we were led to the conclusion 

 that microscopical measurements made in this way were very accurate. lu fact we 

 are inclined to believe that microscopical measurements may be made more 

 accnrately and, considering the accuracy, more expeditiously, than in any other 

 wa}' cxcept by the use of a filar ocular micrometer, which nnfortunately was not 

 available for this work. 



A point which needs con.sideration in connection with the method of making 

 measurements is the (juestion of symmetry of the shells. Wliile it is known as 

 a matter of common Observation that Arcella shells are in general circular in 

 outline, it is not so certain whether they are all exactly circular. We found in 

 examiniiig a large number of shells that there are a cousidcrablo number which 

 are not exactly circular but are .slightly ovatc in outline. With practice an 

 observer becomes very skilful in detecting any deviation from the circular outline, 

 and in taking our measurements whenever it was suspected that any such deviation 

 was prcsent in a shell two pairs of diameters (inner and outer) at right angles to 

 one another were measured instead of a single pair. These diameters were so 

 taken as to be, as nearly as one coidd judge, the maximum and minimum diameters. 

 Then the average of the two was takon as represeuting what the diameter of this 

 shell wüuld be were it .symmctrical. All these deviations observed were small — in 

 most cases extremely small. No strikingly asymmetrical shells were found in the 

 course of this work. 



In taking the individuals for measuremcnt there was no selective choice 

 exercised. Soiue water was drawn up from the culture in a pipette and dropped 

 on a slide. If there were any Arcellae in the water they were all measured. If 

 none were found a new .sample of water was taken. The sample of the population 

 measured was a " random sample." 



In Order to test quantitatively the generally accepted view that there is a con- 

 sidcrable degree of correlation between darkness of colour and size in Arcella 

 Shells it was desii-ed to collect Statistical data on the colour of the shells. Tho 



