156 WHEELER. 



EXPLANATION OF PLATE IV. 



{Xiphidhwi ensifericm.) 



Fig. 32. Transverse section through the head of an embryo in the stage of 

 Fig. 5 (E) PI. I. X 230. /i^., labrum; /r.' {o.g.), pc.,^ pc.,^ first, second and third 

 protocerebal lobes ; o.p., optic plate ; en. ms., mesentoderm. 



Fig. 33. Next following section to that represented in Fig. 32. X 230. n.b., 

 neuroblast ; am., amnion. Remainmg letters same as in Fig. 32. 



Fig. 34. Next following section to that represented in Fig. 33. X 230. st., 

 stomodasum. Remaining figures the same as in Figs. 32 and 33. 



Fig. 35. Transverse section through the labrum, in a stage intermediate be- 

 tween E and F (Figs. 5 and 6, PI. I.) X 230. pc.,^ third protocerebal lobe ; lb., 

 labrum ; en. ms., mesentoderm. 



Fig. 36. Transverse section through labrum and brain of an embryo in Stage 

 F (Fig. 6, PI. I.) X 230. Ib.y labrum ; pc.,^ pc.,^ pc.,^fv[&t, second and third proto- 

 cerebral lobes ; op., optic plate ; nb., neuroblast ; db., dermatoblasts ; coe., head- 

 ccelom ; 7ns., mesoderm cells ; igl., intraganglionic thickening ; st., stomodaeum ; 

 a7n., amnion. 



Fig. yj. Transverse section through praelabral region of an embryo in stage 

 somewhat later than F (Fig. 6, PI. I). X 145. mc, median cord ; pc.^ {o.g.), first 

 protocerebal lobe (optic ganglion) ; %u., orifice of involution of the intraganglionic 

 thickening. Remaining letters as in Fig. 36. 



Fig. 38. Transverse section through optic ganglion and optic plate of an em- 

 bryo in Stage G (Fig. 7, PI. I). X 145. th., clear thickening in the optic plate; 

 e,, eye ; on., optic nerve ; pc.^ {o.g.), optic ganglion ; am., amnion. 



Fig. 39. Frontal section through the brain of an embryo somewhat older than 

 I (Fig. 9, PI. I). X 175. /., problematical brain segment; pc.^ {o.g.), optic gan- 

 glion; pc,.^ pc.,^ second and third protocerebral lobes; at., deutocerebrum ; tc, 

 tritocerebrum ; e., eye ; md., mandible ; md.s;., mandibular ganglion ; r.g., recurrent 

 ganglion ; st., stomodasum. 



Fig. 40. Transverse section of brain through the supraoesophageal commis- 

 sure of an embryo in Stage K (Fig. 10, PI. I). X 175, nb., neuroblasts; igl., 

 interganglionic thickening ; th., clear thickening in the optic plate ; coe., head- 

 ccelom ; ps., Punktsubstanz ; 071., optic nerve. Remaining letters as in Fig. 39. 



Fig. 41. Transverse section from same series as that represented in Fig. 40, 

 but passing through the frontal ganglion. X 175. dc, deutocerebrum ; f.g., 

 frontal ganglion ; sb., subcesophageal body. Remaining letters as in Figs. 39 

 and 40. 



