Embryology and Growth of Fishes.— The egg of the fish develops only after fertilization, the union of its nuclear substance with that of a sperm-cell from the male. When this process, known as amphimixis, takes place, the egg is ready to begin its segmentation. The eggs of fi s allfishes contain more or less of food-yolk at tachsd to the structures of the germ-cell proper and included with it in the same spherical mass. The presence of this food-yolk affects the man ner of segmentation.
The process of cell-division or segmentation common among fishes need not be described in detail, as it is essentially that of the higher vertebrates. When the food-substance or yolk is consumed, and the little fish is able to shift for itself, it leaves the egg-envelopes and is said to be hatched.
The young fish usually differs from the adult chiefly in size and proportions. The eye is larger, the head larger, the fins are lower, the appendages less developed, and the body more slender in the young than in the adult. But to most of these distinctions there are numerous exceptions, and in some fishes there is a change so marked as to be fairly called a metamor phosis. In such case the young fish in its first condition is properly called a larva. The larva of the lamprey (Petromyzon) is nearly blind and toothless, with slender head, and was long supposed to belong to a different genus from the adult. The larva of sharks and rays and of some dipnoans are provided with bushy exter nal gills, which disappear in the process of de velopment. In most soft-rayed fishes the em bryonic fringe, which precedes the development of the vertical fins, persists for a considerable time.
Hybridism is very rare among fishes in a state of nature. Two or three peculiar forms among the snappers (Lutianus) in Cuba seem fairly attributable to hybridism, the single sped men of each showing a remarkable mixture of characters belonging to two common species. Hybrids may be readily made in artificial im pregnation. among those fishes with which this process is practicable. Hybrids of the different salmon or trout usually share nearly equally the traits of the parent species.
The age, of fishes is seldom measured by a definite period of years. Most of them grow as long as they live, and apparently live until they fall victims to some stronger species. It
is reputed that carp and pike have lived for a century, but the evidence needs verification. Somes fishes, as the salmon of the Pacific (Oncorhynchus) have a definite period of growth (usually four years) before spawning. After this act all the individuals die so far as known — without exception.
Fishes differ greatly in tenacity of life In general, fishes of the deep seas die at once if brought near the surface. This is due to the reduction of pressure. This forces the stomach out through the mouth and may burst the air bladder and the large blood-vessels. Marine fishes usually die very soon after being drawn out from the sea. Some fresh-water fishes are very fragile, dying soon in the air, often with injured air-bladder or blood-vessels. They will die even sooner in foul water. Other fishes are extremely tenacious of life. The mud-min now (Umbra) is sometimes plowed up in the half-dried mud of Wisconsin prairies. The re lated Alaska black-fish (Dania) has been fed frozen to dogs, and has escaped alive from their stomachs after being thawed out. Many of the catfishes (Ameiurus) will live after lying half dried in the dust for hours. The dipnoan (Lep idosiren) lives in a ball of half-dried mud dur ing the dry season, and the fishes, mostly Asiatic, which possess an accessory breathing organ, can long maintain themselves out of water.
Some fishes can readily resist starvation, while others succumb as readily as a bird or mammal. The limits of distribution of many fishes are marked by changes in temperature. Few marine fishes can endure any sudden or great change in this regard, although fresh water fishes adapt themselves to the seasons. Those fishes which are tenacious of life and little sensitive to changes in climate and food are most successfully acclimatized or domes ticated. The Chinese carp (Cyprinus carpio) and the Japanese goldfish (Carassius auratus) have been naturalized in almost all temperate and tropical river-basins. Within the limits of clear, cold waters, most of the salmon and trout are readily transplanted. But some of these, as the grayling,. are very sensitive to the least change in conditions. Most of the catfish (Silitridce) will thrive in almost any fresh waters except those which are very cold.