of the Fishery Board for Scotland. 



Ill 



may be aggregated towards one pole to form the blastodisc, and the large 

 globe of colourless homogeneous fluid yolk of crystalline transparency — a 

 drop of clear fluid surrounded by the periblast and envelopes (Fig. 15, 

 PI. I.). In some cases one or more oil-globules may be present in the 

 fluid yolk ; in other cases part or all of the fluid yolk is enclosed in large 

 compartments, that is to say it is divided up into segments by delicate 

 protoplasmic partitions or films as in the eggs of the sole, sprat, pilchard, 

 and anchovy. This condition is produced when part of the interstitial 

 protoplasm between the yolk-spherules remains dipping in from the 

 cortical layer; usually it is transferred to the latter. In all cases the 

 spherules are dissolved and the yolk is fluid. No trace of the germinal 

 vesicle or of the nucleoli can be seen, and although we know from the 

 subsequent history of the fertilised egg that the chromatic substance that 

 goes to form the polar bodies and the egg-nucleus must be present in the 

 germinal layer in the neighbourhood of the micropyle, I have been unable 

 to detect it. Thus practically the whole internal fabric of the egg has 

 dissolved. 



That this remarkable change in the pelagic egg at the close of its ovarian 

 growth, by which it becomes fitted for fertilisation and pelagic life, is 

 produced by the entrance of a large quantity of watery fluid will be 

 immediately evident. 



The enlargement and translucent appearance of ripe pelagic eggs 

 have long been known, but the fact that the transformation is 

 due to the imbibition of fluid has hitherto escaped the attention of 

 observers. Thus, for example, Ryder,* speaking of the eggs of the cod, 

 says : — " Such partly developed ova, when examined with reflected light, 

 " appear whitish instead of a clear, transparent, yellowish tint, such as 

 " would be noticed in ripe eggs. This difference in colour is due to a 

 " change in the character of the plasma enveloping the germinative vesicle, 

 " for immediately that the eggs are mature and ready to leave the intra- 

 " ovarian cavity they acquire a remarkable transparency. This must be 

 " due to a comparatively sudden blending of the protoplasmic corpuscles of 

 " the egg into a homogeneous material very like the white or colourless 

 "albumen of a hen's egg." Agassiz and Whitmant speak of the " yolk 



elements having melted together in the form of a homogeneous ball," 

 and Whitman elsewhere % mentions the " clearing up of a pelagic fish egg 



the moment it comes in contact with water, owing to the dissolving of its 

 " opaque granules," but he is describing what occurs in the fully mature 

 egg after it is shed,§ and the disappearance of the granides in the 

 periblast. M'Intosh and Prince, in their elaborate memoir on the 

 development of Teleosteans, state that the translucency of the ripe egg 

 " is due to the disappearance of the granules in the yolk of the ovarian 

 egg " ; II nor does Raffaele, in his memoir on pelagic eggs, advance any 

 explanation of the change.^ Cunningham, in his excellent work on the 

 natural history of marine fishes, mentions the yolk " globules as having 

 run together into one large globe." He also says, " If the roe of a 

 " spawning fish is examined it is seen that some of the eggs have become 

 "larger and transparent, while a portion of the roe still consists of the 

 " chalk- white smaller eggs. In fact, the eggs become more or less 

 " clear and transparent when they are ripe, and then they become loosened 

 " from the sides of the roe, fall into the cavity, and so make their way to 

 * Op. cit., p. 69. 



t Mem. Mus. Gomv- Zool. Harvard College, Vol. xiv. No. /., Part ii,, 1 , p. 15., 

 1889. 



X Journ. of Morphology, Vol. i., p. 227. 1887. 



§ In Ctenolabrus, vide Mem. Mtis. Conip. Zool. Harvard Collexje, Op. cit., p. 11. 

 II Trans. Roy. Soc. Edin. Vol. xxxv. Part iii., p. 668. 1890. 

 if MittJieil. aus der Zool, Stat, zu Neapel. Bd. viii., p. 4. 1838. 

 H 



