of the Fishery Board for Scotland. 



123 



The Relation of the Change in the Pelagic Ovum to Certain 

 Phenomena in the Life-History of the Species. 



In the floating egg itself it may be noted that the aggregation of 

 germinal matter or blastodisc, and the embryo, are placed inferiorly owing 

 to the lower specific gravity of the diluted yolk. Reference has also been 

 made to the sinking of the embryo or larvse as the yolk is absorbed, from 

 the increasing specific gravity of the tissues into which it is transformed, 

 and to the correlation of the inferior nutritive power of the diluted yolk 

 and the characters of the larva). In cases where the yolk consists of solid 

 plates, as among Elasmobranchs, or semi-solid spherules, as among most 

 demersal eggs, the apparatus or arrangement by which it is rendered 

 available for assimilation is necessarily more complicated than in pelagic 

 eggs, where the heart of the embryo opens directly into the fluid yolk, 

 and where the absorption is a comparatively simple process. Comparison 

 of the larvae which issue from demersal and pelagic eggs shows the 

 diff'erence in the nutritive power of the yolk in the two cases. Thus, 

 the egg of Cottus scorpius is of about the same size as the egg of the 

 plaice — the yolk alone, indeed, is somewhat smaller. The former hatches 

 in between three and four weeks. The larva is about 7*5mm. in length, 

 and has the mouth open, and is in other respects well developed. The 

 egg of the plaice at the same mean temperature hatches in about sixteen 

 days ; the larva is about 4*lmm. long, delicate and slender, destitute of a 

 mouth, and is otherwise ill developed. 



Thus a group of facts associated with the embryonic and larval stages 

 is explicable by a knowledge that the yolk is diluted in pelagic eggs. 



Another group relates to the adult condition. The rapid expansion of 

 the ovum to three or four times its previous size makes it physically 

 impossible for a female producing pelagic eggs to carry all her eggs in the 

 mature distended state, because the volume of the ripe eggs may 

 approximate to or exceed the volume of the body of the fish. Fishes 

 whose eggs are pelagic are far more fecund than those whose eggs are 

 demersal.* I have computed the mean number produced annually by 

 each female of twelve species with demersal eggs to be 24,700, while the 

 mean number produced by each female of thirty species with pelagic eggs 

 is 2,388,000. This excessive fecundity, while dependent for the most 

 part on the vicissitudes to which a wandering egg is exposed, is also, no 

 doubt, related to the imperfect character of the embryo when born. Some 

 species could possibly carry all their eggs if the mature condition was the 

 opaque stage before hyaline expansion occurs, and they were then extruded. 

 The mean gross volume of the eggs produced each season by a female in a 

 number of species is nearly as follows — the calculation being based upon 

 the average volume of the individual egg and the number of eggs 

 produced: — Plaice, 1129 cubic centimetres; cod, 6233 c.c. ; haddock, 

 718 c.c; turbot, 4723 c.c; halibut, 86,196 c.c. A female cod, 32J 

 inches in length, and weighing 141b. loz., had a volume of 6250 c.c. 

 The ovaries, which contained fully matured eggs, and were probably partly 

 spent, weighed 31b. 3oz., and possessed a volume of 1682 c.c, so that 

 the volume of the body of the fish, minus the ovaries, was 4568 c.c. It 

 was calculated that the number of eggs present was over 4,550,000, 

 which, in the mature distended state, would occupy a volume of about 

 6261 c.c, or considerably greater than the bulk of the fish. In the 

 flounder, again, which is probably the most fecund of sea fishes, the 

 contrast is still more striking. A very large female examined in 



* Vide Comparative Fecundity of Sea Fishes. Ninth Annual Reportf Fishery Board 

 for Scotland. Fart Hi., p. 243. 



