No. 3] EGG OF ALLOLOBOPHORA FOETIDA. 613 



photograph, let the microscope stand undisturbed for at least 

 ten minutes before pulling down the camera, and never attempt 

 to take a photograph unless the focus has held absolutely true 

 during this interval. After the photograph is taken, raise the 

 camera and again examine the preparation to see that the 

 focus has held while the plate was being exposed. Atten- 

 tion should be given to the working condition of the microm- 

 eter screw ; it should be tight enough to preclude any chance 

 of slipping after the focus is finally adjusted. 



A special test must be made for each magnification, but this 

 need not mean undue experimenting, for two magnifications 

 should be adequate for practical use, the lower for photo- 

 graphing entire sections, and the higher for special details, 

 such as centrosomes. Restricting one's photographs, as far 

 as possible, to one magnification, offers a great advantage for 

 comparison of shrinkages.^ 



We feel sure that our photographs in this paper are merely 

 a promise of better results. It has been necessary in the 

 reproduction to sacrifice many minor details to bring out a few 

 important points. Our original prints are partly responsible 

 for this, for in them, to insure a successful reproduction of 

 the salient points in a section, we sacrificed the general cyto- 

 plasmic structure. For example, we have experimented with 

 photo. 17, and have made a much stronger impression of the 

 cytoplasm, and at the same time have not obliterated the 

 middle-piece in the aster by over-printing. 



1 We have taken a photograph of a stage micrometer at the two magnifica- 

 tions we use, 660 and 950. A print from this negative gives a scale by which any 

 detail in the photograph can be readily measured. 



EXPLANATION OF PLATES. 



All the photographs, except Nos. i, 8, 9, 23, 35, 36, and 37, were taken in the 

 winter of 1899, on the Lumiere (France) or Nye (Belgium) plates. 



In three or four cases a detail in the original print has been slightly strength- 

 ened with a lead-pencil, merely enough to overcome in part what is lost by 

 reproduction. 



If any of our readers should wish to compare the reproductions with the solar 

 prints, one or more may be obtained on request. 



