674 SUMMARY OF CURRENT RESEARCHES RELATING TO 



fluid or chromic acid. Undiluted Miiller's fluid in twenty-four hours gives 

 more satisfactory preparations than a weak solution which is allowed to 

 act for a longer period. Chromic acid 1/5 per cent, for ten or twelve 

 days gave most of the preparations from which the drawings of the nerve- 

 endings in the author's paper were made. A few drops of acetic and osmic 

 acid added to distilled water gave a very energetic macerating fluid for the 

 epithelium of marine molluscs. Such preparations led to the discovery of 

 the very delicate outward continuations of the pigmented cover-cells in the 

 compound eyes of Area. 



If. Arthropods. — In order to demonstrate the presence of the corneal 

 hypodermis in the facetted Arthropod eye, and the connection of the so-called 

 " rhabdom " with the crystalline cone cells, it is necessary to resort to 

 maceration. In most cases it is hardly possible to determine the 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 Miiller'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 greatly 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 depig- 

 mented sections, which should be resorted to in special cases only. 



Infixing the tissues of the eye, it is not sufficient to place the detached 

 head in the hardening fluid ; antennae and mouth-parts should be cut off as 

 close to tbe 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 diminished 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. Eibbon sections cannot 

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

 advantage in order to obtain very good sections. 



Killing Polyzoa.* — Mr. T. Whitelegge writes : — " I place a small twig 

 of Polyzoa in about two or three drachms of water ; when fully expanded 

 I add about two drops of chloroform, and these should be dropped in so 

 that they sink to the bottom. In from a quarter to half an hour I add 

 spirits, about six drops at a time, and stir up gently, so that it gets mixed 

 with the water. The spirits and chloroform stupefy them, and I try 

 touching one to see if they are in a .sleepy condition ; then I add 

 more spirits gradually, mixing it and the water each time. When the 

 fluid consists of equal quantities of water and spirit, I let them stand for a 

 time, then add spirit very cautiously till they are in nearly pure spirit. 

 This is necessary, as they contract, even after death, if the water is extracted 

 from them too rapidly. When they are killed they should on no account 



* Trans, and Auu. Kep. Manchester Micr. Soc, 1886, pp. 30-1. 



