EARLY DEVELOPMENT OF RANA TEMPORARIA. 185 
dorsal one having been separated off whilst the ventral 
still retains its primitive connection with the persistent neural 
sheath. 
The Amphibian embryonic neural sheath may possibly hence 
represent a primitive condition, and the formation from it of 
sense organs and nerves, resembling as it does that which is 
found in certain Invertebrates, may reveal a more ancestral 
process than the method of their formation in Elasmobranchs 
and birds.; 
DESCRIPTION OF PLATE X, 
Illustrating Mr. W. B. Spencer’s Paper on “Some Notes on 
the Early Development of Rana temporaria.” 
Alphabetical List of References. 
al, Alimentary canal. az. Anus. 4/. blastopore. ep. Epiblast. ep’. Epi- 
dermic layer. ep". Nervous layer. f. 4. Fore brain. gass. Gasserian ganglion. 
h. 6. Hind brain. Ay. Hypoblast. Jat. Lateral line. mes. Mesoblast. x. 
Pedal nerve. 2’. Lateral nerve. x. c. Neural canal. 2.7. Neural folds. 
n.g. Neural groove. z.7. Neural ridge. xe. c. Neurenteric canal. zo. 
Notochord. o/f. Olfactory organ. op. v. Optic vesicle. sc. Suckers present 
in embryo frog. ys. Yolk. 2. Second head cavity. V. Trigeminal nerve. 
zx. Point of fusion of layers. 
Fies. 1—4 represent external views of embryos of Rana temporaria. 
Fig. 1.—From the dorsal surface. The neural folds have not yet closed, 
and are especially wide open in the head region. 
Fig. 2.—The posterior end of an older embryo, seen from the ventral 
surface. The folds are just meeting, but not obliterating the blastopore. 
Fig. 3.—A similar view of a slightly older embryo. The folds are still 
closer, but the blastopore persists. 
Fig. 4.—Ventral view of a considerably older embryo, in which the tail 
is just beginning to grow and the line of union of the folds is but very 
faintly marked, the blastopore persisting as the anus at its end, 
