No. 3-] EGG OF ALLOLOBOPHORA FOETID A. 6ll 



After photographing, this ovary was immersed for fifty-three 

 hours in turpentine and the same nucleolus again photographed 

 without staining (photo. 36). The fat granules (osmophile 

 granules) had neither dissolved out nor faded in any part of 

 the ovary. This discredits the advice of those authors who 

 recommend placing sections for twenty-four hours in turpen- 

 tine or xylol to remove the fat granules before staining. In 

 some cases, perhaps as a rule, warm xylol or turpentine will 

 fade the blackening caused by osmic, but it is by no means 

 infallible, and, moreover, the xylol very often does not dissolve 

 the fat substance. Photographs have enabled us to discover 

 its presence after a careful examination under the microscope 

 had convinced us it had dissolved out completely. Again, after 

 long immersion in turpentine, and after staining, the granules 

 are still sharply blackened, and confidence in the certainty of 

 their removal by turpentine leads one to misinterpret them as 

 other than fat granules. 



Further details regarding the nucleoli and the ovarian egg 

 will be discussed and illustrated by photographs in a paper 

 now being prepared for press. 



Method. 



Further experiments with the mechanical mode of focussing 

 described in our last paper ^ have led to the development of a 

 simpler and more rapid method of overcoming one of the 

 practical difficulties encountered by the cytologist in photog- 

 raphy. 



After abandoning the effort to focus fine details on the 

 ground glass of the camera, or through a transparent portion 

 marked off in the ground glass, we used the method described 

 in detail in the paper just referred to — ascertaining by experi- 

 ment with a large object, easily focussed on the ground glass 

 (a sharply stained nucleolus, for example), just what difference 

 the pointer on the micrometer screw registered, between the 

 focus through the microscope and the focus on the ground 



1 " Further Notes on the Egg of Allolobophora foetida," Zoological Bulletin, 

 vol. ii, No. 3, 1899. 



