THE EVIDENCE OF THE OLFACTORY APPARATUS 229 
rapidity and thickens considerably, thus forcing the opening of the 
hypophysial tube more and more dorsalwards, until at last, in the full- 
grown Ammoceetes, it becomes the dorsal opening of the nasal tube, 
as already described. Here, then, in the hypophysial tube we have 
the original position of the olfactory tube of the vertebrate ancestor, 
and it is significant, as showing the importance of this organ, to find 
that such a hypophysial tube is characteristic of the embryological 
development of every vertebrate, whatever may be the ultimate form 
of the external nasal orifices. 
The single median position of the olfactory organ in the Cyclo- 
stomata, in contradistinction to its paired character in the rest of the 
Vee 
ee wy ¥ ‘ 4 
r auet eae ae i 
Hy ul Or Il vel 
Fic. 100.—Ganeuia oF THE CRANIAL NERVES OF AN AMMOCMTES, 4 MM. IN LENGTH, 
PROJECTED ON TO THE Mupian Puanu. (After KuprFer.) 
A-B, the line of epibranchial ganglia; aw., auditory capsule; nc., notochord; Hy., 
tube of hypophysis; Or., oral cavity; w.l., upper lip; 1.2. lower lip; vel., septum 
between oral and respiratory cavities; V., VIU., [X., X., cranial nerves; 2., 
nerve with four epibranchial ganglia. 
vertebrates, has always been a stumbling-block in the way of those 
who desired to consider the Cyclostomata as degenerated Selachians, 
for the origin of the olfactory protuberance, as a single median 
plakode, seemed to indicate that the nose arose as a single organ and 
not as a paired organ. 
On the other hand, the two olfactory nerves of Ammoccetes 
compare absolutely with the olfactory nerves of other vertebrates, 
and force one to the conclusion that this median organ of Ammo- 
ccetes arose from a pair of bilateral organs, which have fused in the 
middle line. 
The comparison of this olfactory organ with the camerostome 
