136 W. BALDWIN SPENCER. 
Fics. 5—7 represent transverse sections through an embryo of the age of 
Fig. 1 in the posterior region. 
Fig. 5.—The most posterior section and cut somewhat obliquely through 
the medullary folds, showing the groove between these running to the 
blastopore. 
Fig. 6.—Cuts through the blastopore below and the open neural groove 
above. 
Fig. 7.—A more anterior section shows the groove still open above, and 
the alimentary canal lying in the midst of the yolk-cells. The nervous 
layer is seen continous round the body, and connected with that portion 
which has become much enlarged to form the neural folds. 
Figs. 8—]1.—Transverse sections through an embryo intermediate in age 
between those in Figs. 3 and 4. The folds have closed over and the neural 
canal opens into the blastopore. 
Fig. 8.—The most external and posterior section shows the line of union 
of the folds. 
Fig. 9.—Cuts through the part of the neural canal which opens into the 
blastopore. 
Figs. 10 and 11 are more anterior sections and show the alimentary canal 
and neural canal cut in sections, fig. 10 passing through the blastopore 
below. 
Fics. 12—14 represents sections through an embryo of the age of Fig. 4. 
Fig. 12.—Most posterior section passing through the blastopore. 
Fig. 13.—More anterior section, showing the alimentary canal into which 
the blastopore has opened below, and a line of dark cells leading up to 
the dorsal surface where the neural canal is seen dimly outlined. The 
dark cells represent the open passage of Fig. 9. 
Fig. 14 represents a more anterior section of the same embryo. 
Fie. 15.—A vertical longitudinal section through an embryo of the same 
age as in Fig. 4. The fused layers are seen at x, and the hinder part of the 
nervous system is solid and has lost its opening into az., which corresponds 
to the blastopore which has not closed up. 
Fic. 16.—Transverse section through an embryo of Rana temporaria, 
showing proliferation of the cells of the inner nervous layer of the epidermis 
to form the fifth cranial nerve. On the right side the Gasserian ganglion is 
seen in course of’ formation. 
Fies. 17 and 18.—Transverse sections through a somewhat older embryo to 
show the position of the fifth nerve after separation from the epiblast, except 
at the point where the Gasserian ganglion is formed. 
Fic. 19.—Transverse section through an axolotl embryo, showing the for- 
mation of the fifth nerve on each side. a tae, 
Fic. 20.—Transverse section through a much older embryo of Rana 
