194 BULLETIN OF THE 



That the retinal cells and the post-retinal cells, as well as the pigment 

 cells, contain pigment, has already been stated. Lankester and Bourne 

 ('83, p, 194) were somewhat in doubt whether the retinal cells in the 

 median eyes of scorpions contained any pigment. Patten ('86, p. 728) 

 believes that they do not contain pigment. The evidence furnished by 

 sections perpendicular to the length of the cells CPl. II. fig. 3, gra. pig.) 

 is, I think, conclusive. 



Under the head of "intrusive pigmentary connective tissue," Lankes- 

 ter and Bourne ('83, p. 191) include the pigment cells in Androctonus, 

 and, with less confidence, their so-called intracapsular pavement. The 

 pigment cells proper are considered by them as of mesodermic origin. 

 This they defend by several arguments, but admit that their reasons 

 cannot be regarded as offering a sufficient basis for a final conclusion. 



During early stages in the development of Centrurus, mesodermic 

 tissue is often seen making its way into the substance of the brain, and 

 its appearance is characteristic. It penetrates into the nervous system 

 as thin continuous sheets of cells, which in cross-section appear as lines. 

 During the development of the eye no such appearances have been en- 

 countered, and it is fair to conclude that mesodermic tissue has not 

 gained access to the eye by the same means that it has to the brain. 



Lankester and Bourne suggest that it may have entered the eye 

 capsule at the opening for the optic nerve ; but the capsule (sclera) is 

 reflected on to the optic nerve, and, even admitting that mesodermic 

 tissue did gain access here or from the brain, where it undoubtedly exists, 

 one would naturally expect the pigment to appear first in the region of 

 the optic nerve. Contrary to this, as Kowalevsky and Schulgin ('86, 

 p. 531) have shown, — and my own observations confirm theirs, — pig- 

 ment first appears in the front of the retina on its ventral — afterward 

 becoming its anterior — edge, at a point farthest from that where the optic 

 nerve joins the retina. Taking all the evidence into account, it seems 

 that the nerve-end cells, the intracapsular pavement cells (post-retina), 

 and the pigment cells are alike ectodermic, and that the retina contains 

 no tissue that can be referred to a mesodermic source. 



The optic nerve in the adult scorpion joins the eye at a point on the 

 under side of the eye capsule. From this point bundles of fibres pass 

 anteriorly through the base of the retina in front of the post-retinal 

 layer, and from small secondary bundles are given off single fibres which 

 join the bases of the retinal cells. 



In the youngest stages studied, the optic nerve was already formed, and 

 its fibres (PI. III. fig. 17, n.fbr.) passed over the front of the retina, 



