of the Fishery Board for Scotland, 



117 



These considerations led me in 1891 to state that the change was due to 

 the accession of watery fluid, and the accurate determination of the 

 proportion of water present in the two stages, which Dr. Milroy kindly 

 made at my request, shows that such is the case. In the mature eggs of 

 the plaice, dried to a constant weight at 60°C., 80°C., 100°C., and finally 

 at 105°C., the percentage of water was found to be the very high one of 

 91 "86, while in the opaque eggs treated in the same way the quantity 

 was only 65*5 per cent. Later determinations in other cases will be 

 given by him in his report. (See p. 137.) 



It has been already said that this change in the egg at the conclusion 

 of ovarian growth is not confined to the pelagic forms, but that it also 

 takes place in demersal eggs. I believe this fact to be of great 

 importance and significance. In the pelagic ovum one perceives at once, 

 as a result of the absorption of watery fluid, the diminution in the specific 

 gravity, by which the egg is enabled to float and to become a member of 

 the pelagic fauna. Had the change been peculiar to pelagic eggs, one 

 might have been justified in supposing that the function of the absorbed 

 fluid was simply hydrostatic, and that the concomitants of the process — the 

 solution of the yolk-spherules, and the vanishing of the germinal vesicle 

 — were not related to it. But when the same thing is found to occur at 

 the final stage of maturation of demersal eggs, in which diminution of 

 specific gravity is of no utility, quite a new light is thrown upon the 

 nature and significance of the process. For the fundamental and essential 

 feature of the maturation of the demersal egg is the disappearance of the 

 germinal vesicle and the transference of^^^the chromatic substance to the 

 periblast — a phenomenon which also accompanies the absorption of fluid 

 in the pelagic egg. The conclusion is therefore warranted that the fluid 

 which enters the ovum is associated with the dissolution of the germinal 

 vesicle. And if this is so, then it is extremely improbable that it should 

 be confined to the teleostean egg. It will, I think, be found to occur 

 normally in the maturation of the eggs in other groups — such, for example, 

 as the Mammalia ; in other words, that the transformation of the germinal 

 vesicle, the causation of which has been hitherto obscure and mysterious, 

 is caused by the physico-chemical action of fluid which enters the egg at 

 the close of ovarian growth. 



My investigation of the maturation of the demersal eggs of Teleosteans 

 is not yet completed ; but I have found the absorption of fluid, with the 

 concomitant disappearance of the germinal vesicle, the enlargement of the 

 egg, and the partial " clearing up " of the yolk, to occur in all those which 

 I have recently been able to examine in the ripe condition and in the 

 previous opa(iue stage — viz., the herring, the armed bullhead (Cotfus 

 scorpius), the lumpsucker (Cydopterus lumpus), and the angler (Lophius 

 piscatorius) ; and from the assumption of translucency in other forms 

 there can be no doubt it occurs in all. I have found the same translucent 

 change in the eggs of Cottus bubalis, Liparis montagui, AnarrMchas 

 lupus, Gasterosteus spinachia, Syngnathus acus, and Ammodytes tohianus ; 

 and M'Intosh and Prince,* IIolt,t Cunningham, J and several other 

 writers refer to the translucent appearance assumed by the ripe demersal 

 eggs of many species. 



While the nature of the change is the same both in the pelagic and 

 demersal ovum, the relative quantity of fluid absorbed is much less 

 in the latter than in the former. In the majority of cases, at least, the 

 yolk-spherules are not all dissolved, although many of them flow together, 

 and the vitellus appears more translucent and watery and increases in size. 



* Op. cit. , p. 668. t ScL Trans. Roy. Dublin Soc. , Series ii. Vols. ic. , v, 



X Nat. Hist. Marine Fishes, p. 71, &c. 



