536 



NA TURE 



\Aprn 9, 1885 



as5umed by the older observers that all marine fishes 

 followed similar habits. Thus it was supposed that the 

 co i, haddock, whiting, ling, hake, and other fishes fre- 

 quented certain banks for the purpose of depositing their 

 eggs, and that various flat fishes, such as the larger 

 examples of turbot and sole, came from deep water to 

 shallow water for the same end. Such conjectures, how- 

 ever, were found to deviate very considerably from the 

 actual condition. 



Amongst the earliest to notice that the eggs ot certain 

 marine fishes floated were the cod-fishermen of the Loffo- 

 den Islands, off the coast of Norway. These Norwegians 

 had noticed that what they called the "roe" of the cod- 

 fish floated in the water on the great fishing-banks, and 

 often at certain seasons to such an extent as to make the 

 water thick. Prof. G. O. Sars, Inspector of Fisheries in 

 Norway, to whom this remark was made, supposed that the 

 fishermen had mistaken some of the lower marine animals 

 for the eggs of fishes, for such a feature was in direct 

 opposition to anything he knew of the spawning of fishes. 

 The subject, however, was soon set at rest, for he pro- 

 ceeded in 1S64 to the fishing-grounds above-mentioned, 

 viz. off the Loffoden Islands, and captured in the tow-net 

 immense numbers of the eggs of the cod floating at the 

 surface of the sea. Next year, indeed, on a calm day, 

 Prof. Sars found the sea covered with a dense layer of 

 floating spawn, so that with a sufficiently large net he 

 could have taken tons of it. This occurred over a cele- 

 brated fishing-ground, on which the cod were present in 

 enormous numbers, so as to form what the fishermen 

 called a " fish mountain." Sars also found that the ova 

 of the haddock floated, and amongst the eggs procured 

 from the surface of the sea were some from which young 

 fishes resembling gurnards emerged, and he correctly 

 concluded that the ova of the gurnard followed the same 

 habit as those of the cod and haddock. 



The impetus given to such observations by the ener- 

 getic action of the United States Fish Commission en- 

 abled the Americans to corroborate the discovery of the 

 Norwegians in regard to the floating of the ova of the 

 cod, which lately have been artificially'hatched on a some- 

 what extensive scale on their coasts. The labours of the 

 distinguished Prof. Alex. Agassiz in the same country 

 have further added to our knowledge of floating eggs, so 

 that the number of fishes in which this occurs is con- 

 siderable. Thus the majority of the American flounders, 

 certain kinds of wrasses (Ctenolabrus), a species of spar- 

 ling (Osmcrus), several species of cottus, cod, haddock, 

 gurnard, shad, mackerel, and Spanish mackerel, a kind of 

 dory {Zeus), and the frog-fish are amongst those which 

 have floating eggs. The late Dr. Malm of Gothenburg 

 further increased the list by discovering that the eggs of 

 the plaice were similarly buoyant ; and G. Brook has 

 recently added to this category the eggs of the lesser 

 weever. The very great influence which this floating 

 of the tiny eggs exercises on the multiplication of the 

 food fishes will be apparent as we proceed. 



On the other hand, most freshwater fishes (except the 

 shad) deposit their eggs on the bottom like the salmon, 

 or on water-plants, like the carp and pike : while other 

 marine species, such as the herring, sprat, lump-sucker, 

 and bimaculated sucker, follow a similar method. The 

 number of marine fishes which are supposed to deposit 

 their eggs on the sea-bed is yearly diminishing, while the 

 ranks of those in which the ripe eggs are found to float 

 correspondingly increases. 



To come now to our own shores, and to confine our 

 remarks to what is really the most important group of 

 fishes, viz. the food-fishes, we find that early in spring the 

 surface of the sea over the great fishing-banks, such as 

 Smith Bank, off the north-east of Scotland (Caithness), pre- 

 sents vast numbers of floating eggs of food-fishes, together 

 with multitudes of the very young fishes provided with a 

 yolk-sac exhibiting various degrees of absorption. Some 



of the ova {e.g. those of the haddock and gurnard) are 

 larger than those of the cod, but they are few in number ; 

 while a fourth kind are smaller than any yet mentioned. 

 When placed in a vessel of sea-water the eggs persistently 

 float on its surface, descending but a very little when the 

 jar is rudely shaken. Even after a protracted journey 

 only the dead eggs roll on the bottom of the vessel. All 

 the floating eggs are living. Moreover, the eggs were 

 removed from the cod itself, and carried from Smith Bank 

 to the Marine Laboratory at the harbour. On arrival, 

 these floated at the surface of the vessel. On transferring 

 them to a larger jar and turning on a tap of sea-water, a 

 great change occurred. The ova in a few minutes lay 

 on the bottom. Microscopic examination subsequently 

 showed that the edge of the germinal area was disin- 

 tegrating — free protoplasmic processes and separate cells 

 occurring all round. The cause of this sudden change 

 was doubtless the impurity of the water (for the proper 

 apparatus had not yet been fitted up), the metallic pipe 

 (block-tin) containing an opaque whitish deposit which 

 speedily killed the ova. The addition of methylated 

 spirit in the same way sends all the eggs and embryos to 

 the bottom. Sars, indeed, mentions that if the eggs of 

 the cod are placed in fresh water they sine, and never rise 

 again. They are killed — just as a newly-hatched salmon 

 is killed — though somewhat more slowly, by immersion in 

 sea-water. Sars thinks that even a fall of rain might 

 affect the floating of the ova in the sea, but this is 

 unlikely. 



More than once the eggs of the haddock and other 

 fishes have been brought under notice as lying on the 

 bottom of a vessel, and therefore held as proving that 

 the ova did not float. But in every case such eggs were 

 found to be dead or dying, unripe, or not even fertilised. 

 If in removing the eggs from a fish, too much pressure is 

 applied, unripe eggs escape. Such either sink or float 

 ambiguously, according to the stage of development. 

 Unless this fact is borne in mind, disappointment naturally 

 occurs, especially to one who has triumphantly carried 

 such eggs from deep-sea fishing to vindicate statements 

 that have been impugned. No one ever asserted that 

 dead eggs floated. It is the ripe and living eggs that are 

 so buoyant. 



In the Marine Laboratory it has happened that some 

 living ova of the cod rolled on the bottom of the vessel, 

 but this was clearly due to the attachment of fine par- 

 ticles of mud and sand which had gained admission from 

 imperfections in the temporary apparatus, and which 

 surely and speedily in every case proved fatal to the 

 embryo. 



The ova and embryos brought from the surface of the 

 sea are comparatively hardy, even though kept for ten 

 days without renewal of the sea-water. The lively little 

 cod, about 5 mm. in length, with their characteristic black 

 pigment-patches, swam actively at the surface of the 

 water, darting hither and thither when interfered with, 

 while a stratum of the dead lay at the bottom. The water 

 may even be somewhat milky and the odour characteristic, 

 and yet the embryos survive — until, as Sars also found, 

 the yolk-sac, which supplies them with nourishment, is 

 absorbed. 



The difference between the larval cod and the young 

 salmon just hatched is striking. The former (that is, the 

 young cod) is in a very rudimentary condition, not only 

 in size, but in structure. For instance, the heart pulsates, 

 but, as my colleague, Prof. Pettigrew, observed, there is no 

 visible blood and no blood-vessels. Those, therefore who 

 say that the heart in animals contracts from the stimulus 

 of its living blood, would here find little support. On the 

 other hand, the newly-hatched salmon has attained great 

 complexity : indeed, several days may be spent in de- 

 lineating its elaborate blood-vessels alone. 



{To be continued.) 



