266 AYERS ON THE DEVELOPMENT 



blastosphere, and invagination in so far as it takes place most actively in a re°ion which 

 in other insects is the invaginated tract. It may be said further that the mesoderm and 

 endoderm are at first undifferentiated, since they have the same origin and are to be distin- 

 guished from each other only at a much later period. (Compare Metschnikoff and Ganin 

 loc. cit.) ' 



The elongated embryo is now crescentic in shape with an anterior, oral or cephalic end 

 and a posterior, abdominal region. (PI. 24, figs. 2, 8, 9, 13, 14.) The former is the lamer 

 while the latter is much smaller and tapers gradually to a point. The region of greatest 

 thickness lies about one-fourth of the distance from the anterior toward the posterior end of 

 the body, boon after the embryo assumes its crescentic form it secretes about itself a thin 

 colorless, transparent cuticula, which (in specimens treated with reagents) projects in front 

 beyond the cell mass into a frontal process, and a little behind this on the ventral side into a 

 decided prominence, which bears at its apex the mouth opening. The cuticula of the free 

 edges of the oral opening is thickened to form four crescentic chitinous mandibles. Poste- 

 riorly the cuticula is extended beyond the body mass to form a long, tapering more or 

 less curved caudal appendage. The mesenteron at first ends blindly in front as well as 

 behind, but by coalescing with the invagination of the stomodaeum it soon comes to open 

 out through the mouth The muscular system of the first larva consists of oral, cephalo- 

 thoracic and abdominal muscles. The oral muscles are four in number arranged in two 

 pairs. Of these the lateral pairs are much the larger. (PI. 24, fig. 6.) The shape and size 

 of the muscles vary much in the same larval stage. The oral muscles are fan-shaped 

 and at then- smaller ends are inserted in the thickened bases of the mandibles, from which 

 they extend out to their origin in the lateral and ventral body walls. A cephalo-thoracic 

 flexor and abdominal segmental muscles form the greater part of the muscular system 

 of this larva Some of the abdominal muscles extend into the tail region giving great 

 flexibility to this organ at its junction with the abdomen. The number of bocly segments 



buTZu "l T ge V f eS ^ ifferent krVae - The maXimUm number obser/edi,°ei-h 

 but the usual number is five. The cuticula is thickest in the head region ; here it is seen 

 in optical section, with a magnifying power of 600 diameters, as a d°oubl -outline 1 ayeT 

 InLe7. tant - ^ but gradually merging into a single line in the abdominal region 

 In the equator of each segment the cuticula is produced into numerous, finely pointed hol- 

 low bristles, the lumen of each of which is at least partly filled with a finely 8 L alar pro- 

 toplasm which contains a nucleus situated opposite the mouth of the lumen. On the venteal 

 side of the base of the caudal appendage there are to be seen a varying number of ho ow 

 tooth-like projections* of the cuticula also filled with a finely gramilar protop asm Th 

 caudal appendage ends bluntly, and its terminal fourth is covered by nunerou fine brist 

 which are somewhat restricted to its dorsal surface This appendage, like he teeth 

 at its base and the frontal lobe, is filled in the living animal with a gi^u a pro opllsm 

 containing few if any nuclear elements. The hypodermis, which lies* immediate^ unde" 



n^z^ v ^zz^r^:i^x^ ^f-tr ionsofthe -«. »-* , * e caudal 



could only have been executed by the action of TuohT, *PP enda gf- ^ey arose respectively from the cuticula of the 



' One p'ecuhar larva W as observed (p,?,,!^ n -^ J™ h6ad ' t *' ^'^ "* *°" *» ™* *** 



besides the bristles upon each segment, there were tiee Iar™ JUSt ^ ^ '^ ° f tU ° audal a PP^*ge. 



