FORMATION OF GEBMINAL LAYEBS IN CHELONIA. 245 



EXPLANATION OF PLATES XIV, XV, XVI, & XVII. 



List of Bcference Letlers. 



ao. Area opaca. ap. Area pellucida. am. Amnion, a. Line along which 

 nuclei arc specially heaped up in the germinal wall (Figs. 18 and 20). hi. 

 Blastopore. W. ^j. Blastoporic passage, c/t. Notochord. f-ci. Ectohlast. CH.Ento- 

 blast. e/(c. Chorda entoblast. entL Darm entohlast. ^. h'. Germinal wall. /(./. 

 Head-fold. mes. Mesoblast. n. Nuclei in the yolk. si. " Sickle." v. o. Ventral 

 opening of the blastoporic passage, yk. Yolk, yk, c. Yolk-corpuscles, yk. p. 

 Yolk-plug. z. Shelf-like extension into the archenteron. 



All the figures, excepting Figs. 5, 6, lia, 16, and 35, have been drawn by 

 C. Ishikawa. Figs. l--() and 35 have been re-drawn by M. Indö. 



Figs. 1 — 4 have been drawn with Zeiss's A A, x 2; Figs. 7 — 17 with Zeiss's 

 C, X 2; Figs. 18—81 with Zeiss's D D, x 2; Fig. 35 with Zeiss's B B, x 2; 

 Fig. 5 not drawn to scale ; Fig. 6 is diagram. 



Fig. la. — Dorsal view of the embryonic shield from an egg just deposited. 

 Fig. \h —Ventral view of the same. 



Fig. 2a. — Dorsal view of the eroibryouic shield from an egg laid forty-eight 

 hours. 



Fig. ih. — Ventral view of another embryonic shield of the same age from the 

 same deposit. 



Fig. 3. — Dorsal view of the embryonic shield from an egg laid thirty-six hours. 

 Fig. ia. — Dorsal view of an embryo from an egg laid three days. 

 Fig. ih. — Ventral view of the same. 



Fig. 5. — Ventral view of an embryo from an egg laid five days. 

 Fig. 6. — Diagram of the embryonic shield. 



Figs. 7 — 15. — Series of transverse sections of the embryonic shield given. in 

 Figs, la and h, arranged from behind forward. 



Fig. 7. Section of the region where the three germinal layers are free from 



one another. 

 Fig. 8. Section of the primitive streak. 

 Fig. 9. Section passing directly behind the blastopore. 

 Fig. 10. Section passing just in front of the dorsal lip of the blastopore. 

 Fig. 11. Section through the blastoporic passage. 

 Fig. 12. Section passing through the posterior part of the ventral opening 



of the blastoporic passage. 

 Fig. 13. Section passing through the anterior part of the ventral opening of 



the blastoporic passage. 

 Figs, li and 15. Sections passing in front of the ventral opening of the 



blastoporic passage. 

 Fig. 11a. Section showing; the shelf-like extension into the archenteron. 



