EXPLANATION OF PLATES 1V & V.—continued. 
becoming marked off from the alimentary cavity. 
figs. 4 and 5. x 
Fic. 6.—Part of a transverse section of an embryo of the same age as 
that illustrated in fig. 4. The section passes through the rudiments of the 
two gills, gg. Other letters as m-figs. 3, 4, 5. 
Fic. 7.—Part of another transverse section from the same specimen. 
Letters as in the preceding figures. 
Fic. 8.—The central portion of a transverse section of an embryo about 
the same age as that of fig. 5. 7¢s. Extremity of ink-sac. g. Root of 
gill. ov. Vascular spaces. (f. Space filled with food-yelks. ym. Yelk 
epithelium. Ap. Hepatic diverticula of the alimentary canal, a /. ; 
Fie. 9.—Part of a right and left longitudinal section of a similar embryo. 
H. Part of the circulatory organs. s. Space left between the yelk-— 
epithelium and the intestinal epithelium, due to the contraction caused by 
the hardeuing reagents. Other letters as in preceding figures. 
Fic. 10.—Left side of a longitudinal right and left section of an embryo 
Loligo of the same age as that of fig. 4. ym. Yelk-epithelium. mes. Meso- 
blast. ep. Epiblast. w@. Nerve-ganglion. 2. Retina. oc. Primitive 
optic chamber. ws. White body. mf Mantle-flap or free margin. 
ps. Pen-sac. dg. Median groove, indicating the line of closure of the lips 
of the pen-sac. 
Fig. 11.—A similar section of a much more advanced embryo. ep. Epi- 
blast. mes. Mesoblast. ym. Yelk epithelium. w@. Nerve-ganglion. 
wB. White body. joc. Primitive optic chamber. soc. Secondary optic 
chamber. 4? Iridean folds. of. Otocyst. g. Gill. zs. Ink-sac. ai. Ali- 
mentary canal (rectum). m¢f. mantle-flap. 
. Fie. 12.—Side view of eye-rudiment in a specimen of same age as that of 
g. 3. 
Fic. 13.—View of same from above. 
Fic. 14.—Similar view of a somewhat later condition. 
Fic. 15.—Diagrammatic vertical section of fig. 13. ep. Hpiblast. 
mes. Mesoblast. ym. Yelk epithelium. . Wall of the primitive optic 
chamber, poe. 
Fie. 16.—Similar section of fig. 14. 7. Commencing iridean fold. 
or. Orifice of invagination of the primitive optic chamber not quite 
closed. 
Fic. 17.—Primitive condition of the otocyst as an open pit or invagina- 
tion of the epiblast. 
Fie. 18.—Section of the eye in a more advanced state (from preparation 
preserved in Canada balsam). The drawing in this case does not glve a 
section passing through the central point of the eye, but one a little above 
it. iF. Iridean fold. pe. Posterior wall of the secondary optic cham- 
ber, soc. poc. Primitive optic chamber. cz. Ciliary body. 7. Lens. 
R. Retina, 
Fic. 19.—Actual median section of the front part of the same specimen, 
more highly magnified. D. Line of demarcation between the epiblastic 
cell-lining of the primitive optic chamber and its mesoblastic Investment, JZ. 
Lt. Lens. &. Retina. ez. Ciliary body. soc. Secondary optic chamber. 
poc. Primitive optle chamber. 7/. Iridean fold. 
Fic. 20.—Median section of a part of the eye of a more advanced 
embryo. rpg. Retinal pigment. Other letters as in figs. 18 and 19. 
Fic. 21.—A similar section from an embryo nearly re-’v-to leave the 
ege-shell. 70. Tegumentary overgrowth, which e so-called 
‘cornea ” (never completely closed in the middle pc 1y Cephalo- 
poda) and completes the secondary optic chamber. O°)" stters as in 
preceding figures. 
Other letters as in 
H 
