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 Ammoccetes, 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 



v x v\\ IX x 



Hu ui Or 11 vel 



Fig. 100. — Ganglia of the Cranial Nerves of an Ammoccetes, i mm. in length, 



PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.) 



A-B, the line of epibranchial ganglia; an., auditory capsule; nc, notochord ; Hy., 

 tube of hypophysis ; Or., oral cavity; u.l., upper lip ; l.l. lower lip; vel., septum 

 between oral and respiratory cavities ; V., VII., IX., X., cranial nerves ; x., 

 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 



