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 Ammocoetes 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 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 Ammocoetes 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? 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 Ammocoetes 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, _vel._ Opening 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 {229}rapidity and thickens considerably, thus forcing the opening of the hypophysial tube more and more dorsalwards, until at last, in the full-grown Ammocoetes, 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