Part 24

Fig. 1. Transverse section through the pelvic fin of an embryo of Scyllium belonging to stage P, magnified 50 diameters. bp. basipterygium. br. fin ray. m. muscle. h f. horny fibres supporting the peripheral part of the fin.

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.

Fig. 4. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pectoral fins, to shew how the fins are attached to the body, magnified 18 diameters. br. cartilaginous fin-ray. bp. basipterygium. m. muscle of fin. mp. muscle-plate.

Fig. 5. Transverse section through the ventral part of the trunk of an embryo Scyllium of stage P, in the region of the pelvic fin, on the same scale as fig. 4. bp. basipterygium. br. cartilaginous fin-rays. m. muscle of the fins. mp. muscle-plate.

Fig. 6. Pectoral fin of an embryo of Scyllium canicula, of a stage between O and P, in longitudinal and horizontal section (the skeleton of the fin was still in the condition of embryonic cartilage), magnified 36 diameters. bp. basipterygium (eventual metapterygium). fr. cartilaginous fin-rays. pg. pectoral girdle in transverse section. fo. foramen in pectoral girdle. pe. epithelium of peritoneal cavity.

Fig. 7. Transverse section through the pectoral fin of a Scyllium embryo of stage P, magnified 50 diameters. bp. basipterygium. br. cartilaginous fin-ray. m. muscle. hf. horny fibres.

Fig. 8. Pectoral fin of an embryo of Scyllium stellare, magnified 16 diameters. mp. metapterygium (basipterygium of earlier stage). me.p. rudiment of future pro- and mesopterygium. sc. cut surface of a scapular process. cr. coracoid process. fr. foramen. hf. horny fibres.

Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a nearly ripe embryo of Scyllium stellare, magnified 10 diameters. mp. metapterygium. mes. mesopterygium. pp. propterygium. cr. coracoid process.

EXPLANATION OF PLATES 34-42. (XXII. p. 738.)

LIST OF REFERENCE LETTERS.

a. Anus. ab. Air-bladder. ab´. Aperture of air-bladder into throat. ac. Anterior commissure. af. Anal fin. al. Alimentary canal. ao. Aorta. ar. Artery. au. Auditory pit. b. Brain. bc. Body-cavity. bd. Bile duct. bd´. Aperture of bile duct into duodenum. bl. Coalesced portion of segmental ducts, forming urinogenital bladder. bra. Branchial arches. brc. Branchial clefts. c. Pyloric caæca. c´. Apertures of caæca into duodenum. cb. Cerebellum. cdv. Cardinal vein. ce. Cerebrum: in figs. 47A and B, anterior lobe of cerebrum. ce´. Posterior lobe of cerebrum. cf. Caudal fin. cn. Centrum. ch. Choroidal fissure. crv. Circular vein of vascular membrane of eye. csh. Cuticular sheath of notochord. cv. Caudal vein. d. Duodenum. dc. Dorsal cartilage of neural arch. df. Dermal fin-rays. dl. Dorsal lobe of caudal fin. dlf. Dorsal fin. e. Eye. ed. Epidermis. ep. Epiblast. fb. Fore-brain. fe. Pyriform bodies surrounding the zona radiata of the ovum, probably the remains of epithelial cells. gb. Gall-bladder. gd. Genital duct. gl. Glomerulus. gr. Genital ridge. h. Heart. ha. Hæmal arch. hb. Hind-brain. hc. Head-cavity. hpd. Hepatic duct. hm. Hyomandibular cleft. hop. Operculum. hy. Hypoblast; in fig. 10, hyoid arch. hyl. Hyaloid membrane. ic. Intercalated cartilaginous elements of the neural arches. in. Infundibulum. ir. Iris. is. Interspinous cartilage or bones. iv. subintestinal vein. ivr. Intervertebral ring of cartilage. k. Kidney. l. Lens. lc. Longitudinal canal, formed by union of the vasa efferentia. lin. Lobi inferiores. ll. Ligamentum longitudinale superius. lr. Liver. lt. Lateral line. ly. Lymphatic body in front of kidney. m. Mouth. mb. Mid-brain. mc. Medullary cord. mel. Membrana elastica externa. mes. Mesorchium. mn. Mandible. md. and mo. Medulla oblongata. ms. Mesoblast. na. Neural arch. na´. Dorsal element of neural arch. nc. Notochord. nve. Network formed by vasa efferentia on inner face of testis. od. Oviduct. od´. Aperture of oviduct into bladder. ol. Nasal pit or aperture. olf. Olfactory lobe. op. Optic vesicle. op ch. Optic chiasma. opl. Optic lobes. op th. Optic thalami. or ep. Oral epithelium. ov. Ovary. p. Pancreas. pc. Pericardium. pcf. Pectoral fin. pch. Pigmented layer of choroid. pf. Peritoneal funnel of segmental tube of mesonephros. pfp. Peritoneal funnel leading into pronephric chamber. pg. Pectoral girdle. plf. Pelvic fin. pn. Pineal gland. po. Primitive germinal cells. pr. Mesoblastic somite. prc. Pronephric chamber. prn. Pronephros. pr n´. Opening of pronephros into pronephric chamber. pt. Pituitary body. py. Pyloric valve. pz. Parietal zone of blastoderm. r. Rostrum. rb. Rib. rc. Rectum. s. Spleen. sc. Seminal vessels passing from the longitudinal canal into the kidney. sd. Suctorial disc. 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.