June 6, 1889] 



NATURE 



131 



currents in every direction. Some indeed are captured 

 near the bottom by nets attached to the trawl-beam, while 

 experience with the large net of the St. Andrews Laljor- 

 atory has proved that a great number are carried in mid- 

 water. 



When these glassy eggs issue from the female fish, they 

 are soon fertilized in the surrounding water ; so thit in 

 British waters, at any rate, non fertilization is one of the 

 rarest conditions in these pelagic eggs. It is indeed more 

 likely to happen in the case of the herring, which deposits 

 its eggs in masses on the bottom, or in artificial circum- 

 stances in tanks. The unfertilized ^^g soon becomes 

 opaque and sinks, so that it is readily recognized. 



In this connection I would again refer to the notion not 

 long ago firmly rooted in the minds of many — especially 

 those practically engaged in fishing — that the fishes at the 

 spawning season seek the shallow water in which to deposit 

 their eggs. Now there is little in Nature to support this 

 idea. Shore-fishes, it is true, such as the lump-sucker and 

 sea-scorpion [Cottiis scorpius), do deposit their eggs there 

 (and there cannot be a doubt that some of the masses of 

 eggs thus deposited have been mistaken for those of the 

 food-fishes) ; but the edible fishes proper, such as the cod, 

 haddock, whiting, flounders, and others, appear to produce 

 their eggs just where they happen to be feeding at the 

 season. Their eggs are taken in charge by the ocean 

 generally, and hence are independent of any imaginary 

 protection or privilege pertaining to the shallow waters. 



Moreover it does not follow that the fishes of an inclosed 

 bay ^ will increase of themselves. As in the case of the 

 plaice, in shallow sandy bays, it may happen that most of 

 the large mature female fishes are beyond the limits, the half- 

 grown or immature forms mainly occurring within ; pelagic 

 ova therefore must be borne inward, and still more the 

 pelagic young, while the post-larval stages likewise migrate 

 shorewards ; a counter-migration of the older forms sub- 

 sequently taking place to the deeper water. Such bays, 

 therefore, have to depend for their stock of fishes on the 

 unprotected off shore. If by any chance the latter waters 

 were depopulated, the inshore would seriously suffer."^ 



The minute size of the eggs of all the important marine 

 food-fishes enables a fish like the cod, for instance, to pro- 

 duce an enormous number— probably about 9,000,000, as 

 against the 18,000 to 25,000 of the salmon or the 10,000 to 

 30,000 of the herring, both of which fishes deposit their eggs 

 on the bottom. In the same way the very small eggs of the 

 dab provide for a large annual increase of the species. 



The translucent eggs, which, unless they contain a 

 globule of oil, as in Fig. i, are difficult to see in some 

 instances even in a glass vessel, thus escape (by floating 

 throughout the water) the vicissitudes to which a purely 

 surface-life would expose them, such as the admixture of 

 the surface-water with rain, and the attacks of gulls, ducks, 

 and other forms ; and they also are less at the mercy of 

 the active predatory races living on the bottom, not to 

 allude to the risks of being swept by storms on the beach 

 or captured and destroyed by the ground-rope of the 

 trawler. Nature, indeed, could have devised no method 

 more secure than this for the safe propagation of those 

 valuable fishes which for ages h?ve peopled our waters, 

 and I venture to say, with Prof. Huxley, will perhaps 

 people them for ages yet to come, notwithstanding the 

 persistent efforts of man to annihilate them. 



Some good observers, for example Prof. Ryder in 

 America, have attached much importance to the oil-globule 

 in eggs which are pelagic, but its buoyant influence has 

 been slightly over-estimated, for some contain no oil- 

 globule, while the massive od-globules in the eggs of the 

 salmon and cat-fish have no such effect. They float, as 

 well shown by my friend Mr. Edward E. Prince, Secretary 

 to the Mussel and Bait Committee, solely in virtue of 



^ For ex.a nple, closed by a Fishery Order. 



- This feature wa5 poiiitei ou: in tha R;por. of H.M. Tr.iwling Com- 

 mission, under Lord Dalli.usie. 



their specific gravity, which is somewhat less than that of 

 sea-water. The moment fresh-water is added they sink, 

 as they likewise often do when transferred from a vessel 

 filled at sea into one containing shore-water. 



While immediately after deposition these minute spheres 

 are prone to accident from impurity and sudden changes 

 in the temperature of the water, such would not seem to 

 be the case after development has made some progress. 

 Thus many living eggs will be found in odoriferous vessels 

 brought from sea by the fishermen if the inclosed embryos 

 have reached an advanced stage. Again, while carrying 

 out some experiments on temperature (at the suggestion 

 of Prof. Huxley) during the trawling expeditions, I had 

 occasion to heat a test-tube containing some of the eggs 

 of the flounder, so as to make them rush up and down the 

 vessel most actively. Considerable heat was applied, and, 

 under the impression that the eggs were irretrievably in- 

 jured, the tube was set aside. Some days afterwards, when 

 explaining the nature of the experiment to Prof. Ewart, he 

 noticed motion in the tube, and further examination showed 

 that after all this exposure to heat the little flounders 

 had emerged as usual, and were alternately floating and 

 swimming about in the water. On the other hand, severe 

 frosts are fatal to ova crowded in shallow vessels, in many 

 cases actual rupture taking place ;^ and the same occurs 

 in large eggs, for example those of the cat-fish deposited 

 on the bottom of the vessel. 



Out of the little glassy sphere, after a longer or shorter 

 interval (varying from a few days to a few weeks, according 

 to temperature), comes a minute and nearly transparent 

 fish which at first is often as passive in the currents as 

 the eggs themselves - It soon, however, uses its tail for 

 swimming and its pectoral fins for balancing. Its shape 





j^|sss 



Fig. 2. — Larval ling immediately after hatching. 



is somewhat like that of a tadpole, partly from the large 

 head, but mainly from the great size of the yolk-sac, which 

 contains a store of nourishment on which the little mouth- 

 less creature, about 3 mm. long, sustains itself for a week 

 or ten days. In this respect it somewhat resembles the 

 young salmon, in which a much larger collection of the 

 same food supports it for about six weeks amongst the 

 gravel in the spawning-bed of the river, though a closer 

 scrutiny reveals certain essential differences. Thus the 

 store of nourishment in the yolk-sac of the salmon is 

 taken up by the blood-vessels which branch in a complex 

 manner over the whole yolk, whereas in the young cod, 

 though the heart is present and pulsating, not a blood- 

 vessel at first is seen, and none ever enters the yolk-sac. 

 The absorption of this nourishment therefore must take 

 place by aid of the cells and tissues themselves, and there 

 is nothing specially wonderful in this, when the condi- 

 tion in the endoderm of Hydra and other instances of 

 intracellular digestion are considered. 



It has been mentioned that these minute and most 

 delicate little fishes are nearly transparent, and this is 

 more or less the case throughout, though in the majority — 

 even before they leave the egg — points of pigment appear 

 here and there in the skin, so as to give them a distinctive 

 character. After hatching, these pigment-spots branch 

 out in a stellate manner, thus becoming more evident, 

 and it is found that in most cases each little food-fish has 



» Nature, June i836. 



^ For some years the development of fishes has been studied by able 

 workers; amonRst others, on the Continent, by Gtiite. Kupffer, Hoffman, 

 Henneguy, E. Van Heneden. Osjannikov, and Rafaele ; in America, by 

 Ale.\. Agassiz, Ryder, and Whitman ; while iniur < wn country. Ransom, 

 Kleiu, Cunningham, Prince, and B.-ook have carried out similar researches. 



