_sg._ Segmental or archinephric duct. _sgt._ Segmental tubules. _sh._ Granular outer portion of the sheath of the notochord in the vertebral regions. _smx._ Superior maxillary process. _snc._ subnotochordal rod. _so._ Somatic mesoblast. _sp._ Splanchnic mesoblast. _spn._ Spinal nerve. _spv._ Spiral valve. _st._ Stomach. _st._ Seminal tubes of the testis. _sup._ Suctorial papillæ. _t._ Testis. _th._ Thalamencephalon. _thl._ Lobes of the roof of the thalamencephalon. _tr._ Trabeculæ. _ug._ Urinogenital aperture. _v._ Ventricle. _ve._ Vasa efferentia. _vh._ Vitreous humour. _vl._ Ventral lobe of the caudal fin. _vmt._ Ventral mesentery. _vn._ Vein. _vs._ Blood-vessel. _vsh._ Vascular sheath between the hyaloid membrane and the vitreous humour. _vth._ Vesicle of the thalamencephalon. _x._ Groove in epiblast, probably formed in process of hardening. _y._ Yolk. _z._ Commissure in front of pineal gland. _zr._ Outer striated portion of investing membrane (zona radiata) of ovum. _zr´._ Inner non-striated portion of investing membrane of ovum. I. Olfactory nerve. II. Optic nerve. III. Oculomotor nerve. V. Trigeminal nerve. VIII. Facial and auditory nerves. PLATE 34. Figs. 1-4. Different stages in the segmentation of the ovum. Fig. 1. Ovum with a single vertical furrow, from above. Fig. 2. Ovum with two vertical furrows, from above. Fig. 3. Side view of an ovum with a completely formed blastodermic disc. Fig. 4. The same ovum as fig. 3, from below, shewing four vertical furrows nearly meeting at the vegetative pole. Figs. 5-10. External views of embryos up to time of hatching. Fig. 5. Embryo, 3.5 millims. long, third day after impregnation. Fig. 6. Embryo on the fifth day after impregnation. Fig. 7. Posterior part of same embryo as fig. 6, shewing tail swelling. Fig. 8. Embryo on the sixth day after impregnation. Fig. 9. Embryo on the seventh day after impregnation. Fig. 10. Embryo on the eleventh day after impregnation (shortly before hatching). Fig. 11. Head of embryo about the same age as fig. 10, ventral aspect. Fig. 12. Side view of a larva about 11 millims. in length, shortly after hatching. Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect. Fig. 14. Side view of a larva about 15 millims. long, five days after hatching. Fig. 15. Head of a larva 23 millims. in length. Fig. 16. Tail of a larva 11 centims. in length. Fig. 17. Transverse section through the egg-membranes of a just-laid ovum. We are indebted to Professor W. K. Parker for figs. 12, 14 and 15. PLATE 35. Figs. 18-22. Transverse sections of embryo on the third day after impregnation. Fig. 18. Through head, shewing the medullary keel. Fig. 19. Through anterior part of trunk. Fig. 20. Through same region as fig. 19, shewing a groove (_x_) in the epiblast, probably artificially formed in the process of hardening.