300 Alexander Petrunkevitch 



images in one individual and of the elements of the retina in 

 another. I have therefore applied two other methods. The one 

 consists of choosing two individuals of the same species, having 

 eyes of the same diameter and preparing one for the study of the 

 image, the other for sections through the retina. This is possible 

 and yields good results when one has a large quantity of living 

 spiders. By far the better method, the one which I now use 

 exclusively and upon which the conclusions I have reached in this 

 research are based, consists in preparing the spider in such a way 

 as to obtain from the same individual at the same time the lenses 

 intact for the study of the size of the image and the retina for 

 sectioning. This is perfectly feasible even in young spiderlings 

 although it requires not a little patience and experience. I pro- 

 ceed in the following manner: a spider is killed by a cut across 

 the middle of the cephalothorax so as to allow the fixing liquid to 

 penetrate as rapidly as possible. Thus far I have obtained the 

 best results for the purpose with the picro-formalin mixture of 

 Bouin and my sublimate modification of the Gilson liquid. The 

 cephalothorax is allowed to remain for six hours in the fixing 

 liquid and is then transferred in the usual manner to 70 per cent 

 alcohol. The sternum and the mouth parts are now carefully 

 removed with a fine pair of scissors. Then the cephalothorax is 

 placed in a low dish contaming alcohol and held by the side with 

 forceps while a thin and flexible spatula is carefully introduced 

 between hypoderm and chitm. Pushmg the spatula slowly for- 

 ward it is possible to separate the entire chitinous part of the 

 cephalothorax from the underlying hypoderm with all its muscles 

 and organs. The vitreous bodies of the eyes remain with the 

 retinas attached by the optic nerves. These are now separated 

 from the remaining organs, carefully noted to exclude error and 

 placed in separate dishes -in paraffin in the usual manner. They 

 are then sectioned with a microtom, either parallel to the eye-axis 

 •or perpendicular to it. The sections are depigmented in chlorine 

 gas dissolved in 70 per cent alcohol and after washing stained in 

 Heidenhein's haematoxylin. The eye-lenses of the same individ- 

 ual, which have been removed together with the tergum, in the 

 manner described, are now placed in water and carefully cleaned 



