92 bulletin: museum of comparative zoology. 



thickenings are not definitely circumscribed, but merge insensibly with 

 the rest of the blastoderm. 



Previous to Stage 1 the procephalic lobes meet in the median plane, 

 "where the labral fundament then appears. Before the appearance of 

 the labrura, however, the antennal fundaments evaginate from the pos- 

 terior regions of the procephalic lobes. 



In Stages 1 and 2 (Plate 1, Figures 1 and 2) the lobes continue to 

 increase in area and thickness. 



At Stage 3 either lobe is relatively as thick as is represented in 

 Figure 3, pr'ceb., and in lateral surface views (Plate 2, Figures 9, 10, 

 pi''ceb.) appears as a strongly convex, oval protuberance. 



In Stages 4 (Plate 3, Figure 12, 7>r'ee6.) and 5 (Plate 3, Figures 19, 20, 

 21, pr'ceb.) the procephalic lobes change little except in size, and the 

 median depression between them [sul.) is still distinct. 



In Stage 7 (Plate 5, Figure 30) the depression (sul.) becomes obliter- 

 ated, and the eyes (Plate 4, Figure 24, ocl.) and postantenual organs 

 (Figure 24, o.p'at.) appear. At this stage sections show a pair of gan- 

 glionic fundaments (Plate 4, Figure 28, pr'ceb.), the largest and most 

 anterior in the head, with which the next two pairs eventually unite to 

 form the supra-CBSophageal ganglion of the adult (Plate 8, Figure 51, 

 gn.sti'oe.). 



In other Collembola the procephalic lobes develop in just the same 

 ■way, as may be gathered from Nicolet ('42, Smynthurus), Packard ('71, 

 Isotoma), Lemoine ('83, Smynthurus), and Uzel ('98, Tomocerus). 



Also in Campodea the same course of development is followed (Uzel, 

 '98, Taf. 3, Figures 33-36 ; Taf. 4, Figures 37-42) as well as in Lepis- 

 ma (Heymons, '97"). 



In fact, the simple process described for Anurida characterizes not 

 only Orthoptera (Ayers, '84, Wheeler, '93, Heymons, '95), but also in- 

 sects in general. 



The procephalic lobes of Diplopods and Chilopods develop essentially 

 as in insects and Crustacea, but no detailed comparisons can be made 

 as yet. 



The most interesting considerations concerning the ocular segment of 

 Hexapoda relate to its equivalence with the first segment of Crustacea. 



Viallanes ('87, pp. 98-109) has carefully compared the brain in both 

 classes and found a striking agreement, extending to histological details : 



" Considerons en premier lieu la partie lati'rale du protocer6bron, 

 connue des anatoraistes sous le nora de ganglion optique ; die nous montre 

 d'abord, en allant de dehors en dedans, les parties suivantes: les fibres 



