DEVELOPMENT OF PETROMTZON FLUVIATILIS. 369 



Ventricle, atir. Auricle, vv. Valves. s. v. Sinus venosus. li. t. Liver 

 tubules, m. g. Mid-gut. br. b. Branchial bars. v. r. Ventral ridge, my. 

 Myomere. Zeiss's A, oc. 1, cam. luc. 



Fig. 29. — Transverse section through pronephros of larva of forty-seven 

 days. n. Notochord. m.jo. Muscle-plates, nu. Nucleus, ao. Aorta, a. c. 

 Anterior cardinal, gl. Glomerulus. tub. Tubules, s. d. Segmental duct. 

 bl. c. Blood-corpuscles. /. g. fore-gut. d. m. Dorsal mesentery. Zeiss's D, 

 oc. 1, cam. luc. 



Tig. 30. — -Transverse section through fore-brain of embryo, seventeen days. 

 na. Olfactory epithelium, op. v. Optic vesicle, v.f. b. Cavity of fore-brain. 



Fig, 31. — Transverse section through thalamencephalon of larva of eighteen 

 days. pin. Pineal gland, op. th. Optic thalmi. v.f. b. Cavity of fore-brain. 

 na. Olfactory epithelium. 



Fig. 32. — Transverse section through region of mid-brain of larva of sixteen 

 days. st. Stomodial epithelium. V. g. e. Epiblastic origin of ganglion of fifth 

 nerve, n. Notochord. m. b. Mid-brain. 



Figs. 33, 34, and 35. — A series of sections through the anterior end of head 

 of a larva fifty-two days old, to show the ganglia cells at base of olfactory 

 epithelium, u. I. Upper lip. I. I. Lower lip. t. Tentacles, g. n. Ganglion 

 cells at base of nasal invagination, o. e. Ciliated epithelium lining nasal in- 

 vagination, c. Cerebral hemispheres, v.f. b. Cavity of fore-brain. 



PLATE XXVIII. 



Fig. 36. — Branched muscle-fibres of heart of larva forty-nine days old. 

 bl. c. Blood-corpuscles, m.f. Muscle-fibre cut across. 



Fig. 37. — Transverse section through the hind-brain, showing appearance of 

 white matter and ganglion of fifth nerve, h. b. Hind-brain, st. StomodEeum. 

 V. g. Ganglion of fifth nerve. This section is rather oblique. 



Fig. 38. — Transverse section through hind-brain, showing origin of ganglion 

 of seventh nerve from epiblastic ingrowth. VII. g. Ganglion of seventh 

 nerve, au. Auditory vesicle, f. g. Fore-gut. 



Fig. 39. — Transverse section through fore-brain of larva forty-nine days 

 old, to show superior commissure, pin. Pineal gland, v./.b. Cavity of fore- 

 brain, s. cm. Superior commissure, g.h.l. Left ganglion habenulse. g. h.r. 

 Right ganglion habenulse. op. ch. Optic chiasma. pit. Pituitary body. inf. 

 cavity of iufundibulum. u. I. Upper lip. /. /. Lower lip. t. Tentacles. 

 Zeiss's C, 00. 1, cam. luc. 



Fig. 40. — Transverse section through mid-brain of larva of forty-nine days. 

 i. Iter a tertio ad quartum ventriculum. e. Eye. tr. Trabeculse. v. r. 

 Ventral ridge. Zeiss's C, oc. 1, cam. luc. 



