248 



yolk. The germinal vesicle, nucleoli, and yolk-spherules have disap- 

 peared, and the egg is swollen to three or four times its former volume. 

 The exact changes that occur in the germinal vesicle have not been 

 determined satisfactorily. The elimination of two polar bodies from 

 unfertilised pelagic eggs has been described by Cunningham and 

 Kingsley, while Agassiz and Whitman carefully describe the eli- 

 mination of two polar bodies shortly after fertilisation. Kingsl ey admits 

 the possibility of sperms having been present; and there seems little 

 doubt that the polar bodies are not formed until after fertilisation. 

 But I have found no trace of the germinal vesicle or of its contents in 

 the translucent mature egg. The membrane and nucleoli are dissolved 

 and the karyoplasm diffused, and although the chromatic substance 

 must be transferred to the cortical protoplasm near the micropylel have 

 not been able to detect it. This phase in the ovarian development of 

 the pelagic egg is a striking one, marking the conclusion of growth 

 and the preparation for fertilisation. 



That it is due to the comparatively rapid imbibition of watery 

 fluid is shown by the following circumstances. 



1) The rarity of eggs between the tw^o stages described. Many 

 hundred may be examined before intermediate stages are found: these 

 are usually in the opaque condition, but somewhat larger and clearer 

 and they contain the germinal vesicle. 



2) The great difference in the volume of the largest opaque and 

 the mature translucent eggs. Thus the mean diameter of the largest 

 opaque eggs of the plaice [P. platessa) just before the change occurs 

 is about 1,21 mm and the mean diameter of the translucent eggs 

 is about 1,88 mm (see Figure; ; the respective volumes are therefore 

 0,9275 cubic millimetres and 2,479 cubic millimetres — nearly a 

 fourfold increase. So likewise the increase in the volume of the ovum 

 of the haddock [Gadus aegleßnus) is from 0,5236 cmm to 1,5962 cmm; 

 in the cod [G. morrhua] 0,3817 cmm to 1,3706 cmm; in the turbot 

 [Rhombus maximus) from 0,1795 cmm to 0,5556 cmm; in the halibut 

 [Hippoglossus vulgaris) from 8,18 cmm to 28,732 cmm; and so forth. 

 In as much as the eggs, both opaque and translucent, vary a little in 

 size, and are not perfect spheres, these measurements are necessarily 

 approximate ; but they are founded on numerous measurements, and 

 they clearly indicate the great increase in volume Avhich occurs. In 

 demersal eggs the augmentation of the size is much less. The large 

 opaque eggs of the herring [Clupea harengus) for example, which still 

 exhibit the normal germinal vesicle, have an average diamètre of about 

 1,18 mm and a volume of 0,8602 cmm; while the mature egg^ in -which 



