130 LARVAL OCELLI AND THE PARIETAL EYE. 



Another important difference lies in the method of closing the ganglionic 

 infoldings, which is as follows: in the scorpion, the openings to the two pairs of 

 marginal infoldings lengthen till they merge with each other and with the one in 

 the olfactory lobes. A continuous groove, varying in depth, is thus formed 

 around the sides and anterior margin of the cephalic lobes. The edge of the 

 optic plates projects over the groove forming a thin-walled fold, which repre- 

 sents the beginning of the palial fold, its free margin being the neural crest. 



The margin of the palial fold now advances inward and backward over the 

 outer surface of the forebrain. At the same time the olfactory lobes sink below 

 the surface, and slide backward, underneath the second segment, leaving only 

 a small, median part visible from above. 



As the palial fold advances, the optic plate is rolled inward, transferring the 

 median eye placodes from the outer limb of the fold to the inner. When the 

 placodes have been carried about half-way across the surface of the brain, pig- 

 ment develops in them that may be seen, in surface views, through the overlying 

 ectoderm. (Fig. 16, A.) As the edge of the palial fold moves still farther back- 

 ward, the outline of the two eye sacs becomes distinctly visible. (Fig. 16, B.) 



Finally both sacs merge into a single bi-lobed sac, with a narrow neck, or 

 epiphysis, that opens to the exterior through a small pore, which we shall call 

 the anterior neuropore. (Fig. 18, a.n.p.) 



The neck to the eye sac elongates somewhat, its walls thicken and become 

 lined with chiten. It is still open in young scorpions, and remnants of it may 

 persist through life. (Fig. 43, e.t.) 



The posterior edge of the completed palial fold extends straight across the 

 posterior boundaries of the forebrain. (Fig. 18.) When the latter is bent back- 

 ward onto the haemal surface of the egg, the edge of the fold forms the anterior 

 edge of the cephalo-thoracic shield. (Figs. 17, 43.) 



By the time the eye tube and palial fold are completed, the anterior portion 

 of the palium, that is the part overlying the hemispheres, and the part originally 

 connected with the anterior wall of the inferior lobes, has thinned out and is no 

 longer recognizable. The position it would have, if retained up to that period, 

 is indicated in Fig. 43, pi. 



It is clear that the anterior neuropore in the scorpion represents the point 

 over the forebrain toward which the palial folds converge and finally unite. The 

 pore leads, not only into the proximal end of the eye stalk, but also into the fore- 

 brain vesicle and into the olfactory lobes. Furthermore, it is clear that there 

 is no real difference between this method of forming a parietal, or cerebral eye, 

 and that in vertebrates. In the latter animals, the eye tube usually appears at a 

 relatively later stage, as an outgrowth of the completed palium or roof of the brain, 

 near the place where the anterior neuropore closed. In arthropods, the same 

 final condition is shown, and in addition, all the preliminary steps by which the 

 eyes were transferred from their original position to the brain roof. 



The lateral ocelli lie for a considerable period on the external surface of the 



