OBSERVATIONS ON THE CRANIAL NERVES OF SCYLLIUM, 475 
thin, and passes rather abruptly into the much thickened sides. 
To the top of the thickened portion the fifth nerve is attached 
by a long slender root (fig. 1,v a). This root passes down 
alongside the brain, but not in actual connection with it, widen- 
ing considerably as it does so. Its inner border follows the 
curvature of the brain until reaching a point about half way 
down the sides of the neural canal ; it then turns suddenly out- 
wards, leaving the brain altogether, and forming, as it does so, 
a very conspicuous blunt projection (fig. 1, v3), which is in 
rather closer proximity to the brain than is the part of the nerve 
immediately above it. The trunk of the nerve then passes out- 
wards and downwards, lying just beneath the superficial epiblast, 
between it and the outer wall of the second or mandibular head 
cavity (fig. 1, 2). The whole of the nerve, including its root, 
consist of closely-packed spherical or polygonal cells, which, like 
all nerve cells in the early stages, stain very deeply with osmic 
acid, and differ materially in appearance from the much less 
closely arranged mesoblast cells. 
By stage k the root of the fifth nerve has undergone very 
remarkable changes; as shown in fig. 4, the dorsal attachment 
(fig. 1, va) to the top of the thickened side of the brain has 
disappeared completely, and the nerve is now attached to the 
brain at a point about half way down the side (fig. 3, v (3), 7. e. 
at a point exactly corresponding to the conspicuous projection 
(fig. 1, v 8) already described at the earlier stage. Immediately 
beyond the root of origin the nerve enlarges suddenly, and pre- 
sents a distinct dorsal projection at the base of the secondary root 
of attachment. Although hitherto we have not secondary in 
following all the intermediate stages, and have not yet obtained 
satisfactory preparations of the stages earlier than that drawn in 
fig. 1, yet we feel justified in putting forward the following ex- 
planation of the appearances we have just described, relying 
for our justification partly upon the description given by 
Balfour, and still more on our own observations on the develop- 
ment of the roots of the seventh nerve, which will be described 
immediately. 
Balfour has described and figured the fifth nerve as arising 
at “stage G, near the anterior end of the hind brain, as an 
outgrowth from the extreme dorsal summit of the brain, in 
identically the same way as the dorsal root of a spinal nerve.’”! 
He has further described? how, by the growth of the 
roof of the brain, the nerves of the two sides, which at first, 
are in contact dorsally, shift their position and become widely 
separate. His descriptions and our own somewhat fragmentary 
' Op. cit,, p. 191, and Pl. XTV, fig, 3. 
2 Op. cit., p. 196, 
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