ON THE OEiaiN OF VEETEBEATES PEOM AEACHNIDS. 333 



V. The Fore-brain. — The structure of the fore-brain and 

 its nerves of Scorpio and of Limulus cannot be fully under- 

 stood without a knowledge of the development of the brain and 

 optic ganglia of some forms like Acilius. Here the cephalic 

 lobes consist^ at a very early embryonic period^ of three seg- 

 mentSj each segment bearing two pairs of eyes^ a pair of optic 

 ganglia, and a segment of the brain. Each optic ganglion 

 arises from a separate invagination on the median edge of the 

 optic plate, just opposite the pair of eyes to which it belongs^ 

 (Fig. 9, A). 



After carrying the ganglion inwards the invagination closes, 

 leaving the ocelli in their original position on the outer surface 

 of the optic plate (Fig. 5, a). 



The cephalic lobes of Scorpio represent merely a modi- 

 fication of the Acilius type. In both cases the relative posi- 

 tions of the optic plate, optic ganglia, and segments of the brain 

 are, at first, the same (Pis. XXIII, XXIV, figs. 1 — 4). Accord- 

 ing to my interpretation, the ganglionic invaginations of the 

 first segment unite with each other across the median ventral 

 line to form a deep transverse furrow (Fig. 5, a), the thickened 

 walls of which are probably derived from the rudiments of 

 that part of the optic plate, optic ganglia, and brain belong- 

 ing to the first segment. It is possible that this furrow 

 contains an unsegmented portion, comparable with the preoral 

 lobe of Annelids ; but this is a question which cannot be dis- 

 cussed here. No eyes are developed in this segment. 



The second ganglionic invagination is at first like the 

 corresponding one in Acilius, except that the optic plate is 

 rudimentary and the eyes are not at first discernible. The in- 

 vagination becomes so large that it involves the optic plate, 

 which then forms the outer wall of a ganglionic sac (Fig. 

 5, b) ; the crescent-shaped openings to the invaginations, mean- 

 time, move backward and inward until they unite with each 

 other over the median line, forming a single sac with a poste- 



' The optic ganglion to the convex eyes of Vespa, in which no larval 

 ocelli are developed, arises in a similar manner on the median edge of the 

 optic thickening. (See "Eyes of Vespa," Patten.) 



VOL. XXXI, PART III. NEW SER. Z 



