Fig. 10. Section through an ovum, slightly younger than fig. 1. Shewing the primitive cumulus and the columnar character of the cells of one half of the blastoderm. Fig. 11. Section through an embryo of the same age as fig. 2. Shewing the median thickening of the blastoderm. Fig. 12. Transverse section through the ventral plate of a somewhat older embryo. Shewing the division of the ventral plate into epiblast and mesoblast. Fig. 13. Section through the ventral plate of an embryo of the same age as fig. 3, shewing the division of the mesoblast of the ventral plate into two mesoblastic bands. Fig. 14. Transverse section through an embryo of the same age as fig. 5, passing through an abdominal segment above and a thoracic segment below. Fig. 15. Longitudinal section slightly to one side of the middle line through an embryo of the same age. Fig. 16. Transverse section through the ventral plate in the thoracic region of an embryo of the same age as fig. 7. Fig. 17. Transverse section through the procephalic lobes of an embryo of the same age. _gr._ Section of hemicircular groove in procephalic lobe. Fig. 18. Transverse section through the thoracic region of an embryo of the same age as fig. 8. Fig. 19. Section through the procephalic lobes of an embryo of the same age. Fig. 20_a_, _b_, _c_, _d_, _e_. Five sections through an embryo of the same age as fig. 9. _a_ and _b_ are sections through the procephalic lobes, _c_ through the front part of the thorax. _d_ cuts transversely the posterior parts of the thorax, and longitudinally and horizontally the ventral surface of the abdomen. _e_ cuts the posterior part of the abdomen longitudinally and horizontally, and shews the commencement of the mesenteron. Fig. 21. Longitudinal and vertical section of an embryo of the same age. The section passes somewhat to one side of the middle line, and shews the structure of the nervous system. Fig. 22. Transverse section through the dorsal part of the abdomen of an embryo of the same stage as fig. 9. EXPLANATION OF PLATE 33. (XX. p. 714.) Fig. 1. Transverse section through the pelvic fin of an embryo of _Scyllium_ belonging to stage P[5], magnified 50 diameters. _bp._ basipterygium. _br._ fin ray. _m._ muscle. _h f._ horny fibres supporting the peripheral part of the fin. [Footnote 5: I employ here the same letters to indicate the stages as in my "Monograph on Elasmobranch Fig. 2. Pelvic fin of a very young female embryo of _Scyllium stellare_, magnified 16 diameters. _bp._ basipterygium. _pu._ pubic process of pelvic girdle (cut across below). _il._ iliac process of pelvic girdle. _fo._ foramen. Fig. 3. Pelvic fin of a young male embryo of _Scyllium stellare_, magnified 16 diameters. _bp._ basipterygium. _mo._ process of basipterygium continued into clasper. _il._ iliac process of pelvic girdle. _pu._ pubic section of pelvic girdle.