2 28 THE ORIGIN OF VERTEBRATES 



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 lar- 

 val 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 Ammoccetes according to Kupffer's researches, for 

 he finds that when the young Ammocoetes 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 consider- 

 able rapidity, so that we may regard the transformation which takes 

 place at this stage, as in some degree comparable with the great trans- 

 formation which occurs when the Ammoccetes 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 Paheostracan to the Cephalaspid. 



What is the nature of this transformation process as described 

 by Kupffer ? 



It is characterized by two most important events. In the first 

 place, up to this time the oral chamber has been cut off from the 

 respiratory chamber by a septum — the velum — so that no food could 

 pass from the mouth to the alimentary canal. At this stage the 

 septum is broken through, the oral chamber communicates with the 

 respiratory chamber, and the velar folds of the more adult Ammocoetes 

 are left as the remains of the original septum. The other striking 

 change is the growth of the upper lip, by which the orifice of the nasal 

 tube is transferred from a ventral to a dorsal position. Fig. 100, 

 taken from Kupffer's paper, represents a sagittal section through an 

 Ammoccetes 4 mm. long; l.l. is the lower lip, u.l. the upper lip, and, 

 as is seen, the short oral chamber is closed by the septum, rel. Open- 

 ing ventrally is a tube called the tube of the hypophysis, Hy., which 

 extends close up to the termination of the infundibulum. On the 

 anterior surface of this tube is the projection called by Kupffer the 

 olfactory plakode. At this stage the upper lip grows with great 



