842 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



Oral epithelium, ov. Ovary. p. Pancreas, pc. Pericardium, pcf. Pectoral fin. 

 / ch. Pigmented layer of choroid. pf. Peritoneal funnel of segmental tube of meso- 

 nephros. pfp- Peritoneal funnel leading into pronephric chamber, p g. Pectoral 

 girdle, pi/. Pelvic fin. pn. Pineal gland, po. Primitive germinal cells, pr. 

 Mesoblastic somite, prc. Pronephric chamber, prn. Pronephros. prn'. Opening 

 of pronephros into pronephric chamber, ft. Pituitary body. py. Pyloric valve. 

 p z. Parietal zone of blastoderm, r. Rostrum. rb. Rib. re. Rectum, s. Spleen. 

 -s c. Seminal vessels passing from the longitudinal canal into the kidney, s d. Suc- 

 torial disc. sg. Segmental or archinephric duct, sg t. Segmental tubules, sh. 

 Granular outer portion of the sheath of the notochord in the vertebral regions, s mx. 

 Superior maxillary process. s nc. Sub-notochordal rod. so. Somatic mesoblast. 

 sp. Splanchnic mesoblast. sp n. Spinal nerve, sp v. Spiral valve, st. Stomach. 

 st. Seminal tubes of the testis. sup. Suctorial papillae, t. Testis. th. Thalamen- 

 cephalon. thl. Lobes of the roof of the thalamencephalon. tr. Trabeculse. ug. 

 Urinogenital aperture, v. Ventricle, v e. Vasa efferentia. v h. Vitreous humour. 

 v 1. Ventral lobe of the caudal fin. v mi. Ventral mesentery, vn. Vein. vs. Blood- 

 vessel, v sh. Vascular sheath between the hyaloid membrane and the vitreous 

 humour, v th. 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 1 . 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. 



EXPLANATION OF PLATES 3442. 

 PLATE 34. 



Figs, i 4. Different stages in the segmentation of the ovum. 

 Fig. T. Ovum with a single vertical furrow, from above. 

 Fig. i. 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. ii. Head of embryo about the same age as fig. 10, ventral aspect. 



Fig. 12. Side view of a larva about n millims. in length, shortly after hatching. 



Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect. 



