Fish are water-dwelling vertebrate animals that breathe through gills (comb-like tissues filled with blood vessels) that help promote the interchange of waste gases in the body for oxygen in the water. Some fish live only in fresh water, some in salt water and others are estuarine and can live in both fresh and salt waters. Fish with notochords and skulls, but without lower jaws, were common in the Silurian period, 425 to 405 million years ago but their descendants possessing lower jaws were the dominant life forms on our Earth in the Devonian Period, 405 to 345 million years ago.

Most teleost fish are oviparous, that is, they lay eggs in the water where they are fertilized by sperm produced by males. Soon after eggs are laid in water, they absorb water and become hardened. Cleavage soon begins and a blastoderm is formed. As cellular cleavage of the blastoderm continues, the blastoderm spreads over and around the stored yolk materials.

The baby fish shown in this Web site start out as eggs laying on bottom sands, in a nest of some kind, floating around in the open water, or elsewhere. Baby fish first absorb stored yolk materials while they are still in the egg shell and for an additional period of time after they hatch. The early development of fish before and after hatching is extremely complex and can not be fully explored here. The state of development of both structure and form at emergence from the egg and at successive intervals thereafter differs from one species to another.

The illustrations, above right, give a very brief and simplistic view of the early development of fish. Fish usually start out as eggs (Figures 1 and 2). The next illustration (Figure 3) shows a baby fish free of the egg shell, showing mainly yolk materials, a body (former blastoderm) consisting of myomeres (future muscles), and optic vesicles (future eyes). Through gradual changes, the next illustration (Figure 4) shows a swimming embryo, showing mainly primordia (anlagen) of fins, myomeres, eyes and a working mouth. The last illustration (Figure 5) shows a swimming baby fish, although the fins are not yet complete. Among the important anatomical structures in the development of fish embryos are the following: yolk sacs, finfolds, fins, mouths, myomeres, pigmentation and other things.

Some baby fish are altricial while others are precocial. Altricial means "requiring nourishment" and refers to a pattern of growth and development in organisms which are incapable of moving around on their own soon after hatching or being born. Some baby fish hatch, such as sunfish, remain on nests while others, such as cod, remain floating in water for some period of time while they absorb their abundant yolk materials. Precocial fish hatch and develop without a large yolk and are immediately mobile and somewhat able to defend themselves against predators.

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