228 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 Ammoccetes 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 Palostracan 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 Ammoceetes 
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 
Ammoceetes 4 mm. long; /./. is the lower lip, wd. the upper lip, and, 
as is seen, the short oral chamber is closed by the septum, vel, 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 
