The development of the larva takes place with considerable rapidity. The yolk becomes absorbed and the larva becomes accordingly more transparent. It generally lies upon its side, and resembles in general appearance and habit a minute Amphioxus. It is soon able to swim with vigour, but usually, unless disturbed, is during the day quite quiescent, and chooses by preference the darkest situations. It soon straightens out, and, with the disappearance of the yolk, the tail becomes narrower than the head. A large caudal fin becomes developed.
When the larva is about twenty days old, it bears in most anatomical features a close resemblance to an Ammocœtes; though the histological differences between my oldest larva (29 days) and even very young Ammocœtes are considerable.
The mouth undergoes important changes. The upper lip becomes much more prominent, forming of itself the anterior end of the body ([fig. 47], ul). The opening of the nasal pit is in this way relatively thrown back, and at the same time is caused to assume a dorsal position. This will be at once understood by a comparison of [fig. 43] with [fig. 47]. On the inner side of the oral cavity a ring of papillæ is formed ([fig. 47], or.p). Dorsally these papillæ are continued forward as a linear streak on the under side of the upper lip. A communication between the oral cavity and the branchial sack is very soon established.
The gill pouches gradually become enlarged; but it is some time before their small external openings are established. Their walls, which are entirely lined by hypoblast, become raised in folds, forming the branchial lamellæ. The walls of the head cavities between them become resolved into the contractors and dilators of the branchial sacks. The extra-branchial basketwork becomes established very early (it is present in the larva of 6 millimetres, about 9 days after hatching) and is shewn in an older larva in [fig. 47], br.s. It is not so complicated in these young larvæ as in the Ammocœtes, but in Max Schultze’s figure, which I have reproduced, the dorsal elements of the system are omitted. On the dorsal wall of the branchial region a ciliated ridge is formed, which may be homologous with the ridge on the dorsal wall of the branchial sack of Ascidians. It has been described by Schneider in Ammocœtes.
With reference to the remainder of the alimentary canal there is but little to notice. The primitive hepatic diverticulum rapidly sprouts out and forms a tubular gland. The opening into the duodenum changes from a ventral to a lateral or even dorsal position. The duct leads into a gall-bladder imbedded in the substance of the liver. Ventrally the liver is united with the abdominal wall, but laterally passages are left by which the pericardial and body cavities continue to communicate.
The greater part of the yolk becomes employed in the formation of the intestinal wall. This part of the intestine in a nine days’ larva (67 mm.) has the form of a cylindrical tube with very thick columnar cells entirely filled with yolk particles. The dorsal wall is no longer appreciably thinner than the ventral. In the later stages the cells of this part of the intestine become gradually less columnar as the yolk is absorbed.
The fate of the yolk-cells in the Lamprey is different from that in most other Vertebrata with an equally large amount of yolk. They no doubt supply nutriment for the growth of the embryo, and although in the anterior part of the intestine they become to some extent enclosed in the alimentary tract and break up, yet in the posterior part they become wholly transformed into the regular epithelium of the intestine.
On the ninth day a slight fold filled with mesoblastic tissue is visible on the dorsal wall of the intestine. This fold appears to travel towards the ventral side; at any rate a similar but better-marked fold is visible in a ventro-lateral position at a slightly later period. This fold is the commencement of the fold which in the adult makes a half spiral, and is no doubt equivalent to the spiral valve of Elasmobranchs and Ganoids. It contains a prolongation of the cœliac artery, which constitutes at first the vitelline artery.
The nervous system does not undergo during the early larval period changes which require a description.
The opening of the olfactory sack becomes narrowed and ciliated ([fig. 47], ol). It is carried by the process already mentioned to the dorsal surface of the head. The lumen of the sack is well developed; and lies in contact with the base of the fore part of the brain.