OVARIES IN TELEOSTEAN AND ELASMOBEANCH FISHES. 175 



nucleus. The first eight figures are drawn from ova of 

 Zoarces, figs. 35, oG from ova of Sygnathus_, and fig. 37 

 from an egg of Zeus faber, the John Dory. 



On its first appearance the yolk nucleus has the form of a 

 cap of deeply staining substance applied to one side of the 

 germinal vesicle. At this stage (fig. 6) it is very dense, 

 almost homogeneous, and stains brightly with carmine. The 

 yolk nucleus also stains deeply with hematoxylin and with 

 methyl blue. As the egg grows the yolk nucleus becomes 

 detached from the germinal vesicle, granular protoplasm 

 intervening between the two (fig. 13). Vacuoles can now be 

 seen in the yolk nucleus. With the further growth of the 

 egg the following changes, as seen in figs. 7 — 13 and fig. 37, 

 take place: — (a) The yolk nucleus gradually moves away 

 from the germinal vesicle towards the periphery of the egg. 

 {h) As it does so it alters its form, changing from a hemi- 

 spherical or nearly hemispherical mass to a completely 

 spherical shell, forming a distinct zone in the cytoplasm, 

 (c) The yolk nucleus acquires a spongy structure, which 

 gradually opens out during development. Later, the coarse 

 threads of the sponge-work become disconnected with one 

 anothei'. The yolk nucleus breaks up into small pieces (fig. 

 10), the small pieces disintegrate into granules. Finally, 

 amongst the dissolving granules are found the first minute 

 oil drops (figs. 12, 37). {d) As the j^olk nucleus moves away 

 from the germinal vesicle it loses staining power, approxi- 

 mating more and more to the tone of the surrounding 

 cytoplasm. 



The changes in the yolk nucleus above described were 

 followed in sections of material which had been fixed either 

 in Bles's fluid, Hermann's fluid, formic-formol-alcohol, or 

 picro-formalin. The yolk nucleus cannot be studied in 

 preparations of material fixed in mixtures containing nitric 

 acid, since the latter reagent dissolves it out completely. 

 This may be seen at once on comparing fig. 35 with fig. 36. 

 Both these figures represent a few ova near the edge of the 

 germinal lamella in Sygnathus. The first figure shows 



