The formation of the mesoderm in Teleosts is not definitely understood. It is usually said to arise as a process of 'delamination,' i.e., detaching itself in a mass from the entoderm. Its origin is, however, looked upon generally as of a specialized and secondary character. The mode of formation of the gill-slit of the Teleost does not differ from that in other groups; an evagination of the entoderm coming in contact with an invaginated tract of ectoderm fuses, and at this point an opening is later established. 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. [Illustration: 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. [Illustration: 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. [Illustration: FIG. 101.--Larva of the Sail-fish, _Istiophorus_, very young. (After Lütken.)]