AND ARTHROPODS. 165 



cells, it is necessary to resort to maceration. In most cases it is 

 hardly possible to determine these important points by means of 

 sections alone. 



The ommateum of fresh eyes, treated for twenty-four hours or 

 more with weak sulphuric or chromic acid, or in Muller's fluid, 

 may be easily removed, leaving the corneal facets with the 

 underlying hypodermis uninjured. Surface views of the cornea 

 prepared in this way show the number and arrangement of the 

 corneal cells on each facet. In macerating the cells of the 

 ommateum it is not possible to give any definite directions, for 

 the results vary gready with different eyes, and it is also necessary 

 to modify the treatment according to the special point to be 

 determined. It is as essential to isolate the individual cells as it 

 is to study cross and longitudinal sections of the pigmented eyes. 

 In determining the number and arrangement of the cells and the 

 distribution of the pigment, the latter method is indispensable ; it 

 should not be replaced by the study of depigmented sections, 

 which should be resorted to in special cases only. 



In fixing the tissues of the eye, it is not sufficient to place the 

 detached head in the hardening fluid ; the antennaj and mouth 

 parts should be cut off as close to the eye as possible in order to 

 allow free and immediate access of the fluids to the eye. When it 

 is possible to do so with safety, the head should be cut open and 

 all unnecessary tissue and hard parts removed. With abundant 

 material, one often finds individuals in which it is possible to 

 separate, uninjured, the hardened tissues of the eye from the 

 cuticula. This is of course a great advantage in cutting sections. 

 The presence of a hard cuticula is often a serious difficulty in 

 sectioning the eyes of Arthropods. This difficulty can be dimin- 

 ished somewhat by the use of the hardest paraffin, and by 

 placing the broad surface of the cuticula at right angles to the 

 edge of the knife when sectioning. Ribbon sections cannot be 

 made with very hard paraffin, but it is often necessary to sacrifice 

 this advantage in order to obtain very good sections. 



