In the course of the further growth of the young Lepidosteus, the yolk-sack is rapidly absorbed, and has all but disappeared after three weeks. A rich development of pigment early takes place; and the pigment is specially deposited on the parts of the embryonic fin which will develop into the permanent fins.
The notochord in the tail bends slightly upwards, and by the special development of a caudal lobe an externally heterocercal tail like that of Acipenser is established. The ventral paired fins are first visible after about the end of the third week, and by this time the operculum has grown considerably, and the gills have become well developed.
Fig. 68. Head of an advanced larva of Lepidosteus. (After Parker.)
ol. openings of the olfactory pit; sd. remains of the larval suctorial disc.
The most remarkable changes in the later periods are those of the mouth.
The upper and lower jaws become gradually prolonged, till they eventually form a snout; while at the end of the upper jaw is placed the suctorial disc, which is now considerably reduced in size ([fig. 68], sd). The “fleshy globular termination of the upper jaw of the adult Lepidosteus is the remnant of this embryonic sucking disc.” (Agassiz, No. [92].)
The fin-rays become formed as in Teleostei, and parts of the continuous embryonic fin gradually undergo atrophy. The dorsal limb of the embryonic tail, as has been shewn by Wilder, is absorbed in precisely the same manner as in Teleostei, leaving the ventral lobe to form the whole of the permanent tail-fin.
Bibliography.
(92) Al. Agassiz. “The development of Lepidosteus.” Proc. Amer. Acad. of Arts and Sciences, Vol. XIII. 1878.