290 Alexander Petrunkevitch 



By a comparison of all these tables we may now gain consider- 

 able light on the question as to what happens to cephalothorax 

 and eye-group during the post-embryonic development. When 

 the spiderling sheds its first skin in the cocoon, the eye-group 

 occupies almost the w^hole breadth of the cephalothorax which 

 is comparatively very low. The first change, of which I shall 

 speak farther on, is in the directions of the eye-axes. When the 

 spiderling leaves the cocoon, that is, before the next moulting takes 

 place, the eye-axes have become fixed in the positions which they 

 will occupy during the whole life of the growing and mature spider. 

 Whatever change takes place from the time the spiderling leaves 

 the cocoon, is only in the relative size of the space on the cephalo- 

 thorax occupied by the eye-group and to a certain extent, in the 

 relative sizes of the eyes themselves. The cephalothorax groius 

 more rapidly than the eye-group, so that the latter occupies with 

 each moulting a relatively smaller part of the cephalothorax. 

 We shall presently see that this also is of advantage to the spider. 



THE MAXIMUM ANGLE AND THE FIELDS OF VISION 



While handling under the microscope a dried out cephalo- 

 thorax from which all organs and muscles had been removed to 

 permit of the study of the endoskeleton, I chanced to notice that 

 the faint images, visible in the eyes, of the trees which grow before 

 my laboratory window, were not of the same size. I very soon 

 found that boiling or even keeping in a cold solution of potassium 

 hydrate so changes the optical property of the eye-lens that it 

 becomes entirely intransparent so that it is necessary to use some 

 other method. I have finally adopted the following one. The 

 spider is killed in strong alcohol from which it is at once removed. 

 The abdomen, all appendices and the sternum are then removed 

 and the organs and muscles filling the cephalothorax are care- 

 fully taken out with a forceps. Next the inside of the cephalo- 

 thorax must be cleaned under water with a soft brush. I use a 

 small camel's hair brush of the kind used for water colors but cut 

 the hairs quite short. With this brush it is possible to remove 

 all the remaining muscles as well as the vitreous bodies of the 



