EARLY DEVELOPMENT OF RANA TEMPORARILA. 125 
I have, however, in Rana, been unable to find any trace of 
this closure, and in fig. 4 is given a view of the ventral surface 
of an older embryo, in which the tail is just beginning to grow 
out, and the line of union of the folds is but faintly marked, 
whilst at the end of the latter, and occupying the same position 
as in figs. 2 and 3, is yet the blastopore. 
At this stage the line of union may or may not be traceable, 
but the aperture retains the same relative position, and as 
growth proceeds remains open, and can be traced through still 
older specimens until the adult stage is reached. 
This gradual transformation is shown by external views in 
figs. 1—4, whilst it is fully confirmed by the series of sections 
in figs. 5—15. 
Figs. 5, 6, and 7 represent transverse sections through an 
embryo of the age shown in fig. 1, whose neural folds are yet 
open. 
Fig. 7 is the most anterior section, and shows the open 
neural groove on the dorsal surface, while the alimentary canal 
is cut in section in the middle of the yolk. 
Fig. 6 represents a more posterior section through the neural 
groove dorsally and the blastopore ventrally, while in fig. 5 
the two folds are cut through as they pass round the posterior 
end of the egg. (The section is oblique, and hence more is cut 
through on one side than the other.) The sections show 
that the neural groove runs right back to the blastopore. 
Figs. 8—11 are from similar sections of an older embryo 
(such as fig. 3 represents), whose neural folds have met, their 
line of union being seen in the most posterior section (8), while 
one (9), just anterior to this, cuts through the posterior part 
of the neural canal which runs round the hinder part of the 
egg to open at the blastopore. Owing to the direction in which 
it runs a transverse section, such as is represented in fig. 9, 
cuts through a considerable part of its length. 
The two anterior sections show the neural canal dorsally 
and the alimentary canal ventrad of this. 
After this stage has been reached a further important de- 
velopment takes place; the hinder part of the neural canal, 
