BY DR. KLEIN. 153 



The most useful preparations are those made with needles. 

 The carefully excised fresh eye of a frog, newt, rabbit, ox, 

 calf, or pig is divided into an anterior and posterior half. 

 The latter is placed for from twenty-four to forty-eight hours 

 in a one-tenth per cent, solution of perosmic acid, in the dark; 

 thence it is transferred to distilled water for twenty-four hours. 

 After this period small portions of the retina are snipped off 

 and teased in a drop of nearly saturated solution of acetate 

 of potash and mounted in the same fluid. The frog's retina 

 in particular is extremely valuable for the study of the rods 

 and cones with their outer and inner portions, the radial fibres, 

 the nuclei of the outer and inner granular layers, and the nerve 

 fibres and ganglion cells, all of which are much better seen 

 than in retinas which have been macerated in Miiller's liquid. 

 When the object is to study the relations to each other of the 

 different strata in the retina, either of the two following pro- 

 cesses may be employed : 



1. The posterior half of the bulb (or, when small, the whole 

 bulb, after two or three punctures have been made in it), is 

 placed in a two per cent, solution of perosmic acid in the dark 

 for twenty-four hours : it is then removed, and small, oblong 

 pieces are cut from it with a razor (these including, of course, 

 besides retina, corresponding portions of sclerotic and choroid), 

 and placed in alcohol for twenty-four hours or more, until they 

 have attained sufficient consistence for sections to be made 

 from them after embedding. The sections should be mounted 

 in acetate of potash as before. This method answers very 

 well for the retina of the rabbit, calf, or pig. 



2. The other plan, which must also be looked upon as a 

 good one, is the treatment with Miiller's liquid. The entire 

 bulb of one of the above-mentioned animals is placed in this 

 liquid, having previously been punctured at two or three 

 points. After from three to five weeks it is taken out, and cut 

 into an anterior and a posterior half. From the portion of 

 retina belonging to the latter, an oblong piece is removed 

 with fine, sharp scissors (it is generally pretty easy to do this 

 without involving the sclerotic and choroid, since the retina 

 has usually become more or less separated from the latter by 

 the action of the fluid), and transferred for a few days to ordi- 

 nary spirit. From this it is put into dilute carmine solution 

 for twenty-four hours, then washed in acidulated water, and, 

 finally, after half an hour's or an hour's immersion in absolute 

 alcohol, is embedded in the manner previously described 

 (Chapter YL). The sections are transferred in the manner 

 there indicated from the razor to the object-glass, on which, 

 after proper treatment, they are to be mounted in Dammar. 



A skilful manipulator can obtain good results with this 

 method. Yery thin sections show, in a sufficiently clear man- 



