THE EMBRYOLOGY OF A SCORPION. 139 



edges of the hypoblast, aud tlie primitive thickening is well seen in the caudal 

 segment. X iy~- 



Fig. 17. — Surface view of embryo, with three somites fully formed. 

 X ^. 



Fig. 18. — Transverse section through posterior end of Fig. 17, showing 

 hypoblastic mass, mesoblast, &c. X i-\^. 



Fig. 19. — Diagrammatic representation of the relative extension of the 

 various layers in an embryo of the stage of Fig. 17. 



PLATE XV. 



Fig. 20. — Surface view of an embryo with seven somites, drawn as if 

 flattened out. a — b and c — d are the planes of the sections figured in Figs. 

 21 and 23. x ^. 



Fig. 21. — Transverse section through one of the posterior somites of an 

 embryo with seven somites {a — b in Fig. 20), showing the three layers, 

 epiblast thinning in centre, and mesoblast thin ; amnion, serous membrane, 

 and ccelomic spaces, x ^;9■. 



Fig. 22. — Transverse section through one of the anterior somites of Fig. 

 20. X H^. 



Fig. 23. — Transverse section through tail-segment (c — d) of Fig. 20, 

 showing the undivided mesoblast and the hypoblastic mass, x ^f^. 



Fig. 24. — Surface view of embryo of nine somites, drawn as if extended. 

 X -^. 



Fig. 25. — Transverse section through head-segment of Fig. 24, showing 

 epiblast thickening to form cerebral nervous system and spreading (e//.), with 

 the amnion beyond the ventral plate, neural groove, thin mesoblast, with small 

 ccelomic space and hypoblast, x ^^. 



Fig. 26. — Transverse section through one of the anterior somites of Fig. 

 24, showing the epiblast very solid where the appendage will develop (ap.) 

 aud form the neural thickening («. ik.) at each side of the neural groove. 

 Mesoblast thick, and ccelom not very evident, x ^y^. 



Fig. 27. — Diagrammatic representation of the relative extension of the 

 various layers in an embryo of the stage of Fig. 24. 



Fig. 28. — Surface view of embryo at Stage I (ten somites) extended in a 

 plane, showing appendages, cheliceral ganglion, stomodajum, &c. x -\°-. 



PLATE XVI. 



Fig. 29. — Longitudinal section of Stage 1 in the middle line, showing 

 dorsal flexure of the embryo, commencement of tail outgrowth, stomodicum, 

 &c. x -''^. 



