The egg of a fish consists of an enveloping membrane containing the yolk or vitelkis. The first step; in the development of the egg is the formation of innumerable cells on the surface of the vitellus,which are closely packed together, and form a new membrane or layer surrounding the vitellus. The next sign of organization is the thickening and condensation of one spot of this new layer. Tho thickened part has an elongated oval shape, and in its center, running longltudinaliy, is a delicate line or furrow. This is the first beginning of the fish. The backbone of the fish is formed around this furrow. The anterior 3xtremity spreads to become the cavity of the brain, and the tail grows from the posterior end. The yolk remains inclosed in the new layer as in a sac ; as the fish grows this sac becomes constricted, so that the upper part of it is taken up into the body of the fish, while the lower part remains hanging out, and is called the umbihcal vesicle, and it is in this condition that the fish is hatched. He is attached to the upper part of tho umbilical vesicle, which, being too heavy for him to move, he remains anchored by it, as it were, at the bottom of the stream, wriggling only his head and tail. The fish is fed by the absorption of the contents of the vesicle which decreases every day as he grows larger. After some days he is large enough to swim about with the vesicle under hira, and, at the end of forty to fifty days, the sac is no longer to be seen, and the fish swims freely about. All fish, however, are not perfect, and oftentimes deformed ones are met with. Sometimes, instead of there being one fish only attached to an umbilical vesicle, there are tv/o ; not two separate ones,but two heads attached to one body,or two bodies attached to one tail, as shown in Figs. 1 and 2. This curious partial duplication of the fish takes place in the egg long before it is hatched,and is due, probably, to a bifurcation of the furrow around which the backbone of the fish is formed. The cells of the thickened oval spot, instead of forming one straight furrow, for some reason or other, form one in the shape of a Y. Two backbones form around the two branches,with two heads, while one tail has to do for both. As far as has been observed, it is always the anterior part which is duplicated. No one body with two tails has been found. The tail remains single while the head and body are doubled ; and this duplication varies from a partial division of the head only to two nearly complete fish, with different brains, and hearts, and stomachs, and whose hearts do not even beat together, though the circulation in the tail must be common to both. On the other hand the head alone may show signs of duplication. One young fish was found in whom this had extended only to the partial division of the 150 head. Of the four eyes, the two middle ones were not completely separated ; they looked something like a figure of 8 on its side. Generally, one of the half fish is larger and stronger than the other, as seen in Fig. 2, and carries the smaller one off" wherever it will,notwithstanding the apparent eflPort of the smaller one to go somewhere else. These double fish are not very common, and as they die after the vitelline sac has been absorbed they are not seen by fishermen. The ratio of these deformed fish to the number of eggs in the hatching troughs was roughly estimated at twenty to twenty thousand, or one in a thousand eggs. But a curious fact proved that the eggs of some fish contained a larger proportion. One large blind trout had a small pond to herself, and was fed daily by food presented to her on the end of a stick. Her eggs were kept apart,and out of about two thousand there were sixteen deformed fish, or one to one hundred and twenty-five eggs. Certain fish would seem to be more predisposed to produce eggs creating these monstrosities, and were we to ask for the cause of this,we should probably have to look for it in some anomaly of the ovary of the fish which produces the eggs. A deformity more common than the double fish, is an apparent curvature of the spine. The fish, instead of being straight, with the umbilical vesicle under him, is curved round so that its tail tarns under, and sometimes touches the under surface of the sao he is attached to. Fig. 3 represents one of these semicircular fish. They are obliged to swim on their side, and move round and round in a circle, or in a spiral, without being able to go straight. These deformities are mentioned and treated by Buckland ill his " Fish Hatching." He there suggests that humpbacked deformity may have been caused by pressure during their transport in the egg state." In the instances mentioned above, however, tlTere was no transportation, the ova being t iken from the fish on the spot. Oat of two thousand salmon ova hatched at Messrs. Dexter Co.'s fish farm, there were no deformities, but in another lot of about the same number there were two double-headed specimens just hatched out.—Americaii Naturalist.
This article was originally published with the title "Monstrosities among Trout" in Scientific American 21, 10, 149-150 (September 1869)