e, each cell carrying its equal share in the function of heredity. When this process takes place the egg is ready to begin its segmentation. The eggs of all fishes are single cells con

o a double-walled cup, known as a gastrula by the pushing in of one side. All the cells of the blastula are very small, but those on one side are somewhat larger than those of the other, and here the wall first flattens and then bends in until finally the larger cells come into contact with the smaller and the segmentation cavity is entirely obliterated. There is now an inner layer of cells and an outer layer, the inner layer being known as the endoblast and the outer as the ectoblast. The cavity of the cup thus formed is the archenteron and gives rise primarily to the alimentary canal. This third well-marked stage is called the gastrula stage; and it is thought to occur either typically or in some modified form in the development of all metazoa, or many-celled animals. In the lampreys, the Ganoids, and the Dipnoans the eggs contain a much greater quantity of yolk than those of the lancelet, but the segmentation resembles that of the lancelet in

. But in the case of some fishes the young hatches from the egg before it has reached such an advanced state of development, and the young looks very different from its parent. It must yet undergo considerable change before it reaches the structural condition of a fully developed and fully grown fish. Thus the development of most fishes is almost wholly embryonic development-that is, development within the egg or in the body of the mother-while the development of some of them is to a considerable degree post-embryonic or larval development. There is no important difference between e

h of a tree. In the free tips of the smallest branches we have represented the various species of animals in their fully developed condition, all standing clearly apart from each other. But in tracing back the development of any kind of animal we soon come to a point where it very much resembles or becomes apparently identical with some other kind of animal, and going farther back we find it resembling other animals in their young condition, and so on until we come to that first stage of development, that trunk stage where all animals are structurally alike. Any animal at any stage in its existence differs absolutely from any other kind of animal, in this respect: it can develop into only its own kind. There is somethin

lampreys, fishes, batrachians, the reptiles, the birds, and the mammals-have descended from a common ancestor that they all develop a notochord, and those most highly organized replace this by a complete back-bone. It is believed that the descendants of the first back-boned animal have, in the course of many generations, branched off little by little from the original type until there came to exist very real and obvious differences among the back-boned animals-differences which among the living back-boned animals are familiar to all of us. The course of development of

s small, perfectly spherical, and enclosed in delicate but greatly distended membranes. The germ disc is especially small, appearing on the surface as an almost transparent fleck. Among the fishes whose eggs float at the surface during development, as of many pelagic Teleosts, e.g., the sea-bass, Centropristes striat

g prior to germination; b, germ-disk after first cleavage; c, germ-disk a

e second; in the latter the third cleavage has been completed. As in other fishes these cleavages are vertical, the third parall

uclei are seen in the layer of the germ, below the plane of the segmentation cavity. These are seen in the surface view of the marginal cells of this stage; they are separated by cell boundaries only at the sides; below they are continuous in the superficial down-reaching layer of the germ. The marginal cells shortly lose all traces of having been separate; their nuclei, by continued division, spread into the layer of germ flooring the segmentation cavity, and into the delicate film of germ which now surrounds the entire yolk. Thus is formed the periblast of t

s margins grow downward, leaving the segmentation cavity apparent. The rim of the blastoderm has become thickened as the 'germ-ring'; and immediately in front of the dorsal lip of the blastopore its thickening marks the appearance of the embry

xt stage the closur

ately the dorsal section of the germ-rim; the c?lenteron under the dorsal lip has here disappeared, on account of the close approximation of the embryo to the periblast; its last remnant, the K

striatus, natural size. (From l

derm, that it acquires its rounded character; its cellular elements then group themselves symmetrically with reference to a sagittal plane, where later, by their dissociation, the canal of the spinal cord is formed. The growth of the entoderm is another instance of specialized development. In an early stage the entoderm exists in the axial region, its thickness tapering away abruptly on either side; its lower

to arise as a process of 'delamination,' i.e., detaching itself in a mass from the entoderm

in other groups; an evagination of the entoderm coming in contact with an invagi

han that of the sturgeon; it is transparent, allowing notochord, primitive segments, heart, and sense-

sh, Xiphias gladius (Li

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 immatur

, Xiphias gladius (L

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 mot

ail-fish, Istiophorus, v

ri, before transformation, being as large as the

nguilla chrisypa Rafine

eves, to changed environmental conditions, wherein one process may be lengthened, another shortened. So, too, the changes from one stage to another may occur with surprising abruptness. As a rule, it may be said the larval

la chrisypa (Rafinesque), called Lept

the briefest space of time; the young fish, hatched often as early as on the fourth day, is then of the most immature character; it is transparent, delicate, easily injured, inactive; within a

, Acipenser sturio (Linn?us

hthys) of Ch?todon sedentarius

ish, Ch?todon capistr

mbrane-bones. In other forms the bones of the head are in the young provided with long spines or with serrations, which vanish totally with age. Such a change is noticeable in the swordfish. In this species the production of the bones of the snout and upper jaw into a long bony sword, or weapon of offense, takes place only with age. The young fish have jaws more normally formed, and armed with ordinary teeth. In the headfish (Mola mola) large changes take place in the course of growth, and the young have been taken for a different type of fishes. Among certain soft-rayed fishes and eels the young is often developed in a peculiar way, being very soft, translucent, or band-like, and formed of large or loosely aggregated cells. These peculiar organisms, long known as leptocephali, have been shown to be the normal young of fishes when mature very different. In the ladyfish (Albula) Dr. Gilbert has shown, by a full series of specimens, that in their further growth these pellucid fishes shrink in size, acquiring greater compactness of body, until finally reaching about half their maximum length as larv?. Afte

rly larval stage of the Headfish, call

arly larval stage, called Molaca

inn?us). Advanced larv

es are brought around to the upper side by a twisting of the cranium and a modification or division of the frontal bones. When the young flounder is hatched it is translucent and symmetrical, swimming vertically in the water, with one eye on either side of the head. After a little the young fish rests the ventral edge

idism, the single specimen of each showing a remarkable mixture of characters belonging to two other common species. Hybrids may be readily made in artificial impregna

century, but the evidence needs verification. Some 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. In Japan and Chi

(adult), Mola mola

e surface. This is due to the reduction of external pressure. The internal pressure forces the stomach out through the mouth an

fish, from the translucent, loosely compacted larva to the sm

head-fish, Selene vomer (Linn?us).

ish (Dallia) has been fed frozen to dogs, escaping alive from their stomachs after being thawed out. Many of the catfishes (Silurid?) will live after lying half-dried in the dust for hours. The Dipnoan, Lepidosiren, lives in a ball of half-dried mud during the arid season, and certain fishes, mostly Asiatic, belonging

hyalocranius Abbott. Family

h, Dallia pectoralis (Bea

Florida while the temperature was still above the frost-line. Those fishes which are tenacious of life and little sensitive to changes in climate and food are most successfully acclimatized or domesticated. The Chinese carp (Cyprinus carpio) and the Japanese goldfish (Carassius auratus) have been naturalized in almost all tem

ina-fish, Ophiocephalus b

t distances. The quinnat-salmon has been thus transferred from California to Australia. It has been found possible to stock rivers and lakes wi

duals taken. In a condition of nature very many of these eggs would be left unfertilized, or be destroyed by other animals. Fishes are readily kept in captivity in properly co

res are thus restored after injury. Sometimes a fish in which the tail has been bitten off will survive the i

bred in Japan), Carassius aura

onstrosities are not infrequent, and sometimes, as in the goldfish, these have been perpetuated to become distinct breeds or races. Goldfishes with telescopic eyes and fantastic fins, and with the green coloration changed to orange, are r

TNO

sed from Dr. Dean's "Fishes Living and Fossil," in which work the details

ondensed from Dean's "Fi