PROGRESS OF MICROSCOPICAL SCIENCE. 
197 
consist of much elongated cells, contrasting strongly with tho 
spherical or oval cells of the posterior roots. 
Early on the fourth day each anterior root consists of a number of 
such processes placed one behind the other, and lying opposite the 
anterior half of a protovertebra. The total length of attachment of 
an anterior root on the fourth day is equal to about half a proto- 
vertebra. 
The anterior roots grow outwards, and early in the fourth day 
join with the posterior roots to constitute the spinal nerves. 
In the cranial nerves no anterior roots have been observed ; but 
as the observations have not been carried beyond the fourth day, and 
certain of the cranial nerves have not been observed at all, no con- 
clusion as to their non-existence is to be drawn from this fact, which 
can only be considered a doubtful confirmation of Balfour’s failure to 
discover anterior cranial roots in Elasmobranchs. 
The facial and auditory nerves have been seen to arise as a single 
outgrowth just in front of the ear ; this speedily divides into an 
anterior part, which runs downwards in front of the auditory vesicle 
and becomes the facial nerve, and a posterior part, which is closely 
applied to the anterior wall of the auditory vesicle and becomes the 
auditory nerve. 
The fifth nerve arises as a single outgrowth on either side, the 
position of which is very constant. The so-called “ hind brain ” 
consists at forty-three hours of an apparently variable number of 
dilatations separated by slight constrictions, and gradually decreasing 
in size from before backwards. These dilatations are well known, 
but appear to possess more constancy than is usually ascribed to 
them ; the most anterior of them is but little smaller than the 
mid brain. From it tho fifth nerve arises in all the specimens 
examined. 
The third, fourth, and sixth nerves have not been observed ; but a 
slight outgrowth from the summit of the mid brain, noticed in two 
specimens only, may prove to be the commencement of the third or 
fourth. 
The olfactory nerves arise towards the end of the third day as 
solid outgrowths from the anterior end of the fore brain, close to the 
median dorsal line, and exactly correspond in mode of development 
and in appearance with the other cranial nerves and with the posterior 
roots of the spinal nerves. They arise at a time when a section 
through tho anterior part of the fore brain transverse to its longi- 
tudinal axis, and passing through the olfactory pits and nerves, is 
almost perfectly circular in outline, and must therefore be described 
as arising from the fore brain itself, and not from the cerebral 
hemispheres, with which they have no connection at first, and which 
are not nearly such prominent objects at the end of the third day as 
they are often described to be. There is no trace of an “ olfactory 
vesicle ” in the early stages. 
This mode of development of the olfactory nerve in the chick 
would seem to be of considerable morphological importance, since, if 
confirmed, any arguments concerning the composition of the skull, 
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