246 



although quite distinct; the ovum becomes more peUucid. the yolk- 

 spherules more separated, perhaps in part dissolved, and the germinal 

 vesicle disappears. In pelagic eggs the process is carried further. The 

 yolk-spherules are entirely dissolved, as is also the interstitial proto- 

 plasm (except in certain cases where strands remain between masses of 

 fluid yolk) ; the egg is distended to three or four times its previous 

 volume, the zona radiata being stretched and thinned out; the contents 

 are rendered of crystalline transparency and the specific gravity is 

 reduced so that the egg floats in ordinary sea water. The egg is at the 

 same time freed from the follicle. 



The detailed results of the investigation, which was made on a 

 large number of species, will appear in the forthcoming Report of the 

 Fishery Board for Scotland, but the chief features may be stated here. 



The opaque ovarian eggs immediately before the change referred 

 to occurs have the following structure, as shown by examination in 

 the living condition and in mounted sections. The egg-capsule is soft 

 and elastic, its thickness varies in difi"erent species but is greater than 

 in the mature state; usually homogeneous in section, or indistinctly 

 fibrillated, it may show distinct fine radial striation (eg. Solea vulgaris, 

 S. lutea). On surface view two patterns are usually exhibited (I) very 

 fine, minute punctation or dotting (2) a much coarser wrinkled, mesh- 

 work or basket-work appearance. The former which may simulate 

 striation in optical section, or the opening of pores on surface view, 

 is limited to a delicate superficial lager, whose presence may some- 

 times be demonstrated by the rupture of the subjacent zona under 

 pressure, the escaping yolk bulging it and stripping it ofl". In macerated 

 mature eggs it may separate spontaneously as a delicate film. There is 

 also within the zona an extremely delicate transparent homogeneovis 

 membrane, closely applied to the vitellus, such as has been described 

 by Vogt in the demersal eggs of Coregotius palea. and Salmo umbla"^ 

 by Ransom in those of the Stickleback'^ and by O ell ach er in those 

 of the trout ^. Scharf falso found it in sections of the eggs of Trigla 

 gurnadus^-. but its existence as a distinct membrane is usually denied. 

 Its presence may occasionally be revealed by pressure on the fresh 

 ovum, usually it is ruptured with the capsule, but sometimes it re- 

 mains intact, and may then be seen as a delicate follicle bounding and 

 retaining the protruding yolk, and emitting it with a rush when the 

 pressure is increased, or, when the pressure is diminished and the yolk- 



- Embryologie des Salmones p. 29. 



3 Phil. Trans. Vol. 157. p. 433. 



4 Zeitschr. f. wiss. Zool. Rd. XXII. Hft. 4. 1872. 



5 Quart. Journ. Micr. Sc. Vol. XXVIII. p, 53. 



