PROGRESS OF MICROSCOPICAL SCIENCE. 
195 
tinctness to those prepared with picric acid. Good results have also 
been obtained from duck-embryos hardened in picric acid. 
Owing to the less compact character of the mesoblast of the head 
and to the absence of protovertebrae, the development of the cranial 
nerves is easier to study than that of the spinal, and will therefore be 
considered first. 
Transverse sections through the hind brain of a forty-three hours’ 
chick show that the cells along the median dorsal line are more 
spherical in shape and slightly smaller than those composing the rest 
of the brain ; also that these spherical cells grow upwards, so as to 
form a conspicuous longitudinal ridge running along the upper 
surface of the hind brain immediately beneath the external epiblast. 
The ridge is traceable along the whole length of the hind brain, 
but is much more prominent posteriorly than it is in front, where it 
gradually disappears. At intervals the ridge becomes more pro- 
minent, and grows out laterally into paired processes. These processes 
are the rudiments of the cranial nerves ; the cells composing them 
are, like those of the ridge itself, small and spherical, and differ 
markedly from both the elongated cells of the external epiblast, and 
the large, loosely arranged, branching and irregularly shaped meso- 
blast cells. 
At forty-three hours the first pair of these processes arises from 
the anterior part of the hind brain ; it subsequently develops into 
the fifth nerve. 
Immediately in front of the auditory involution (which at this 
period is a wide and very shallow pit) a large outgrowth arises on 
either side, from which the facial and auditory nerves are derived. 
A large outgrowth from the median ridge commences on either 
side a short distance behind the auditory pit, and is of considerable 
longitudinal extent, reaching as far back as the middle of the first 
protovertebra. From this outgrowth are developed the glossopharyn- 
geal nerve and the several branches of the vagus. 
The outgrowth of spherical cells from the summit of the neural 
canal, forming the longitudinal ridge above alluded to, is not confined 
to the hind brain, but is continued backwards without any break some 
distance down the spinal cord. In the spinal cord, as in the brain, 
the ridge gives off at intervals paired lateral processes, which extend 
outwards just beneath the superficial epiblast. These processes corre- 
spond in number to the protovertebrae, and are the rudiments of the 
posterior roots of the spinal nerves. Each process has a longitudinal 
extension equal to about half a protovertebra, opposite the posterior 
part of which it is situated. In the case of the first few spinal nerves 
the processes are somewhat larger, and extend back so as to overlap 
the anterior parts of the succeeding protovertebrae. 
This description, it is believed, differs from any previously pub- 
lished account of the development of the nerves in the chick, but 
agrees remarkably closely with Balfour’s * account of the development 
of the nerves, both cranial and spinal, of Elasmobranchs, and is in 
VOL. XVIII. 
‘Phil. Trans.,’ vol. 166, pt. 1. 
P 
