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Length of the Period of Gestation

eggs, time, shown, set and fish

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LENGTH OF THE PERIOD OF GESTATION.

It is impossible to state this either from observation or direct experi ment. It being impracticable, at the time this research was carried on at the Beaufort Laboratory, to isolate the fish in pairs, direct observa tion of the period of incubation was not feasible. All efforts to effect artificial fertilization proved abortive, nor was I ever able to get eggs in early segmentation stages. Thus it is impossible to fix the time of fertilization, which would give the time of the beginning of incubation. Further, it has been impossible to carry any one set of early eggs through to time of hatching (i. e., bursting the egg-shell) and of the complete closing of the body-walls over the diminished yolk-sac as shown in figure 12, plate iv, at or about which time the young are set free from the paternal mouth.

Confronted by these impossibilities, the best that can be done is to make the closest approximation possible from the data at hand. Now it has been shown in the section on the breeding season that the eggs are "laid" under ordinary conditions of weather (i. e., temper ature) between May 18 and 30, and more narrowly between May 20 and 25. There are exceptions, but on the whole the breeding season is seemingly concentrated within narrow limits and the period of incubation has its approximate beginning within these 10 days.

The information at hand is even less definite as to the time of cessa tion of the care of the male parent. The mortality of eggs hatched in aquaria is very great, and with young eggs all may be expected to die. In MacDonald hatching-jars, which have a bowl-shaped base with water admitted at the center of the base, the eggs are lifted by the incoming current, and all sediment, mold spores, and bacteria are carried off by the overflow at the top. It was thought, moreover, that the use of these jars would conserve the life of the eggs in another way. When the yolk-circulation has extended its plexus of blood vessels over the ventral side of the eggs, if these are allowed to stand in one position, shortly a congestion of these ven tral blood vessels is noticed and this is quickly followed by the death of the eggs. In the mouth of the father the constant current of water due to the respiratory actions keeps the eggs free from sediment, bacteria, etc., and presumably keeps them in motion, thus preventing

the congestion referred to. It was hoped that in the MacDonald jars the uplifting action of the water would keep the eggs "dancing" and prevent this trouble. To some extent this hope was realized. But even in the best-handled jars, although there was little conges tion, the eggs turned white and died by dozens and scores, and in no case was it ever found possible to carry any but late eggs on to hatching. Eggs brought in with young nearly ready to burst the shell could with great care be carried through this dangerous period, and if car ried 3 or 4 days further, little trouble was had in bringing them on to maturity. It is interesting to note, however, that there was never any evidence of death due to bacteria, as has been found in the eggs of the toadfish, Batrachus tau. For the details of the difficulties met with in the effort to hatch eggs, the reader is referred to the author's 1916 paper.

However, although no definite time can be set for the liberation of the young from the mouth of the father, this can at least be approx imated as the time when the body-walls have closed up, as shown in figure 12, plate Iv, and the young fish set out to find their own living. In 1907, on July 29, a little catfish was killed and hardened in formalin for 15 hours, at the end of which it measured 81 mm. long over all, while the yolk-slit measured 16.5 mm. long by 2 mm. wide. On August 1, when I left Beaufort, the other fish of this lot in a large aquarium were relatively as active as the adults in Newport River: their babyhood was over.

In 1908 two sets of larvae were under observation at the close of July. The older ones had the yolk covered over and the slit reduced to a mere seam on July 27; the younger ones, however, on the same day had the yolk only half covered. On August 8, the day before my departure, the body-walls of the younger had nearly closed over the yolk, while the older had bellies showing no more protuberance than is to be found in the adult shown in plate 1, figure 1. These younger eggs were plainly of a late laying.

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