The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 98.—Transverse Section through the Olfactory Passage of Petromyzon.

cart., nasal cartilage.

As is seen, it is difficult to see any connection between these folds of olfactory epithelium and the simple tube of the scorpion. But in the nose, as in all other parts of the head-region of the lamprey, remarkable changes take place at transformation, and examination of the same tube in Ammocœtes demonstrates that the elaborate structure of the adult olfactory organ is actually derived from a much simpler form of organ, represented in Fig. [99]. Here, in Ammocœtes, the section is no longer strikingly different from that of the Thelyphonus organ, but, instead, most strikingly similar to it. Thus, again, it is shown that this larval form of the lamprey gives more valuable information as to vertebrate ancestry than all the rest of the vertebrates put together.

Fig. 99.—Transverse Section through the Olfactory Passage of Ammocœtes.

cart., nasal cartilage.

Still, even now the similarity between the two organs is not complete, for the tube in the lamprey opens on to the exterior on the dorsal surface of the head, while in the scorpion tribe it is situated ventrally, being the passage to the mouth and alimentary canal. In accordance with this there is no sign of any opening on the dorsal carapace of any of the extinct sea-scorpions or of the living land-scorpions, such as is so universally found in the cephalaspids, tremataspids, and lampreys. Here is a discrepancy of an apparently serious character, yet so wonderfully does the development of the individual recapitulate the development of the race, that this very discrepancy becomes converted into a triumphant vindication of the correctness of the theory advocated in this book, as soon as we turn our attention to the development of this nasal tube in the lamprey.

We must always remember not only the great importance of a larval stage for the unriddling of problems of ancestry, but also the great advantage of being able to follow more favourably any clues as to past history afforded by the development of the larva itself, owing to the greater slowness in the development of the larva than of the embryo. Such a clue is especially well marked in the course of development of Ammocœtes according to Kupffer's researches, for he finds that when the young Ammocœtes is from 5 to 7 mm. in length, some time after it has left the egg, when it is living a free larval life, a remarkable series of changes takes place with considerable rapidity, so that we may regard the transformation which takes place at this stage, as in some degree comparable with the great transformation which occurs when the Ammocœtes becomes a Petromyzon.

All the evidence emphasizes the fact that the latter transformation indicates the passage from a lower into a higher form of vertebrate, and is to be interpreted phylogenetically as an indication of the passage from the Cephalaspidian towards the Dipnoan style of fish. If, then, the former transformation is of the same character, it would indicate the passage from the Palæostracan to the Cephalaspid.

What is the nature of this transformation process as described by Kupffer?