DEVELOPMENT OF PETROMYZON FLUVIATILIS. 367 



Fig. 4. — Ovum after the invagination is complete, twelve days old, showing 

 the blastopore, bp., at posterior end of the neural ridge, n. r. 



Fig. 5. — Transverse section through ovum of thirty-six hours, ep. Epiblast. 

 s. c. Segmentation cavity, y. c. Yolk-cells. 



Fig. 6. — Transverse section through ovum of forty-eight hours, s. c. Seg- 

 mentation cavity, ep. Epiblast. i/. c. Yolk-cells. 



Fig. 7. — Transverse section through ovum of sixty-seven hours. 



Fig. 8. — Transverse section through ovum of eighty-six hours, showing 

 epiblast gradually thinning out. 



Fig. 9. — Longitudinal section through commencing gastrula, 136 hours. 

 bp. Blastopore, hi/. Hypoblast, y. c. Yolk-cells. m. Mesenteron. s. c. Seg- 

 mentation cavity. 



Fig. 10. — Section through embryo of about the same stage as Fig. 4. 

 bp. Blatsopore. y. c. Yolk-cells, hd. Head. 



Fig. 11, — Transverse section through the body of an embryo just before 

 hatching, seventeenth day. sp. c. Spinal cord. n. Notochord. m. Me- 

 senteron. mes. Mesoblast. s. d. Segmental duct. Zeiss's A, oc. 2, 

 cam. luc. 



Fig. 12. — Transverse section through embryo of thirteenth day. sp. c. Spinal 

 cord. n. Notochord. mes. Mesoblast. m. Mesenteron. y. c. Yolk-cells. 

 Zeiss's A, oc. 2, cam. luc. 



Fig. 13. — Transverse section through embryo of fourteen days. Letters 

 as in Fig. 12. Zeiss's A, oc. 2, cam. luc. 



Fig. 14. — Transverse section through tail of larva twenty days old. sp. c. 

 Spinal cord. n. Notochord. p. g. Solid postanal gut. mes. Mesoblast. bp. 

 Blastopore, d.f. Dorsal fin. Zeiss's A, oc. 3, cam. luc. 



Fig. 15. — Transverse section from the same series as Fig. 14, but posterior 

 to blastopore, d.f. Dorsal fin. mes. Mesoblast. pr. Fused tissue of noto- 

 chord, spinal cord, and postanal gut, or primitive streak. Zeiss's A, oc. 3, 

 cam. luc. 



Fig. 16. — Transverse section of embryo just before hatching, seventeen 

 days, through region of blastopore, bp. Blastopore, sp. c. Spinal cord. n. 

 Notochord. y. c. Yolk-cells. 



Fig. 17. — Longitudinal section of embryo, showing formation of somites. 

 «. Notochord. mes. som. Mesoblastic somites. sp. c. Spinal cord. d.f. 

 dorsal fin. 



Fig. 18. — Longitudinal section of embryo just before hatching, sp. c. Spinal 

 cord. my. Myomere. «m. ^/. Somatopleuric layer of somite. «/?.jt3/. Splanch- 

 nopleuric layer, n. Notochord. Zeiss's A, oc. 3, cam. luc. 



Fig. 19. — A piece of the cartilage of a branchial bar. 



Fig. 20. — A longitudinal vertical section through the tail of a larva twenty- 



