The late embryo of the Teleost, though of rounded form, is the more deeply implanted in the yolk-sac than that of the sturgeon; it is transparent, allowing notochord, primitive segments, heart, and sense-organs to be readily distinguished; at about this stage both anus and mouth are making their appearance.
Fig. 99.—Young Sword-fish, Xiphias gladius (Linnæus). (After Lütken.)
The Larval Development of Fishes.[10]—"When the young fish has freed itself from its egg-membranes it gives but little suggestion of its adult form. It enters upon a larval existence, which continues until maturity. The period of change of form varies widely in the different groups of fishes, from a few weeks' to longer than a year's duration; and the extent of the changes that the larva undergoes are often surprisingly broad, investing every organ and tissue of the body, the immature fish passing through a series of form stages which differ one from the other in a way strongly contrasting with the mode of growth of amniotes; since the chick, reptile, or mammal emerges from its embryonic membranes in nearly its adult form.
Fig. 100.—Sword-fish, Xiphias gladius (Linnæus). (After Day.)
The fish may, in general, be said to begin its existence as a larva as soon as it emerges from its egg-membranes. In some instances, however, it is difficult to decide at what point the larval stage is actually initiated: thus in sharks the excessive amount of yolk material which has been provided for the growth of the larva renders unnecessary the emerging from the egg at an early stage; and the larval period is accordingly to be traced back to stages that are still enclosed in the egg-membranes. In all cases the larval life may be said to begin when the following conditions have been fulfilled: the outward form of the larva must be well defined, separating it from the mass of yolk, its motions must be active, it must possess a continuous vertical fin-fold passing dorsally from the head region to the body terminal, and thence ventrally as far as the yolk region; and the following structures, characteristic in outward appearance, must also be established: the sense-organs—eye, ear, and nose—mouth and anus, and one or more gill-clefts.
Fig. 101.—Larva of the Sail-fish, Istiophorus, very young. (After Lütken.)
