154 WHEELER. 



EXPLANATION OF PLATE IIL 



[Xiphidmm ensiferum.) 



Fig. 21. Median longitudinal section through the head of an embryo over 

 which the envelopes have just closed. X 230. o., stomodasum ; atn., amnion ; 

 s., serosa ; p.o., indusium ; z., pedicel uniting the indusium to the head of tha 

 embryo ; ec, ectoderm ; ms., mesoderm ; vph., vitellophag ; v., yolk. 



Fig. 22. Median tranverse section of the indusium when it has reached half 

 way round the egg. X 230. f.o., inner indusial layer ; aw.,' outer indusial layer ; 

 s., serosa. 



Fig. 23. Three normal cells from the indusium. X 700. 



Fig. 24. Three cells with shrivelled nuclei from the indusium. X 700. 



Fig. 25. Transverse section through basal abdominal region of an embryo 

 passing to the dorsal surface of the yolk. X 230. nb., neuroblasts ; db., derma- 

 toblasts ; atn., amnion ; ms., mesoderm ; ec, ectoderm. 



Fig. 26. Transverse section through first maxillary segment of an embryo 

 passing to the dorsal surface of the yolk. X 230. jig., neural groove ; eti., ento- 

 derm. Other letters as in Fig. 25. 



Fig. 27. Transverse section through the second maxillary segment of an 

 embryo in the stage of Fig. 6, (/") Plate I. X 230. mx.,^ second maxilla 

 (trilobed) ; mnb., median-cord neuroblast ; ps., Punktsubstanz ; vph., vitellophag. 

 Other letters as in Figs. 25 and 26. 



Fig. 28. Transverse section through the mesothoracic ganglion of an embryo 

 in a stage somewhat younger than G (Fig. 7, PI. I). X 230. g.,'^ younger offspring 

 of the neuroblasts ; g.,^ older offspring of the neuroblasts (ganglion cells) ; ecd., 

 ectoderm ; tnc, median cord (" mittelstrang "). Remaining letters as in preceding 

 figures. 



Fig. 29. Sagittal section through nerve-cord a little to one side of the median 

 line. Embryo in Stage G (Fig. 7, PL I). X 175. md.g., mandibular ganglion; 

 mx.g.,^ first maxillary ganglion ; mx.g.^ second maxillary ganglion ; ig.,^ I'g.,- first 

 and second interganglionic depressions ; p-g-,^ first thoracic ganglion ; p-g.,'^ 

 second thoracic ganglion ; mnb., median cord neuroblasts ; mg., their progeny ; 

 itiL, inner neurilemma. 



Fig. 30. Frontal section through the base of the abdomen of an embryo 

 somewhat older than Fig. 6, (F), PI. I. X 175. nb., neuroblasts; mnb., median 

 cord neuroblasts in the intersegmental regions ; p.,^ metathoracic leg ; //., pleuro- 

 podium ; ap.,^-ap.,^ second to fifth abdominal appendages grazed by the knife. 



Fig. 31. Transverse section through the mesothoracic ganglion of an embryo 

 in a stage between Fig. 9 {J) and 10 {K) PI. I. X 230. hy-, hypodermis (product 

 of dermatoblasts) ; nb., neuroblasts ; g.'^, latest progeny of the neuroblasts ; g., 

 ganglion-cells ; hiL, inner neurilemma ; enl., outer neurilemma ; ps., Punkt- 

 substanz. 



