250 



the large opaque eggs of P. platessa have a specific gravity of about 

 1,070 while that of the translucent eggs ranges from 1,025 to 1,026. 

 Considerable variation occurs in the specific gravity, as in the size, of 

 mature eggs of the same species, or even the same individual. Empty 

 capsules of mature pelagic eggs sink in sea-water ; and when a pelagic 

 egg dies it sinks owing to the fluid yolk diff"using out in consequence 

 of the death and retraction of the periblast. 



5) The action of heat and re-agents — the vitellus of the opaque 

 egg becomes solidified and chalky white, that of the mature pelagic 

 e^^ remains fluid and only becomes slightly milky or opalescent. 



6) The proportion of water present. — Dr. T. H. Milroy, of the 

 University of Edinburgh, who is making for the Scottish Fishery Board 

 a detailed investigation of the chemical changes that occur in the 

 growth and maturation of the eggs of Teleosteans, finds for instance 

 that in the mature buoyant eggs of P. platessa, dried to a constant 

 weight at 60° C, at 80° C, at 100° C, and finally at 105° C, the percen- 

 tage of water present is the very high one of 91,86, while in the opaque 

 eggs it is only 65,5. 



The precise nature of the fluid which enters the eggs is being 

 as far as possible ascertained by Dr. Milroy. It seems to be secreted 

 by the granular layer of the follicle at the close of intraovarian growth, 

 and it probably enters by endosmosis, but the immediate exciting cause 

 has still to be determined. Its composition probably corresponds 

 nearly to that of the ovarian fluid which is found bathing the ripe eggs 

 in the cavity of the ovary, and which is expelled with them when 

 they are shed. Very little appears to be known about normal ovarian 

 fluids in any group (vide Hammarsten, Lehrbuch der Physiolog. 

 Chemie, p. 364 [1895]). In Teleosteans with demersal eggs the ovarian 

 fluid sometimes acts as a cementing substance, but in those with pe- 

 lagic eggs it is watery, mingles with the sea-water and disappears, 

 subserving only a lubricating function in spawning. He n s en ^ has 

 described the free ovarian fluid of Gadus morrhua as having a specific 

 gravity of 1,0115 at a temperature of 8,7° C, as being alkaline in re- 

 action and containing an albuminoid; he regards it as probably origi- 

 nating in Graaf's follicles, and he found the quantity to range between 

 14 and 20 per cent of the total volume of eggs and fluid discharged. 

 The quantity of free fluid present in immature ovaries is very small; 

 it increases greatly when spawning begins, and the proportion is largest 

 in ovaries nearly spent. In P. platessa 517 cc of eggs and fluid, gently 

 pressed from ripe females and drained through silk-netting, furnished 



6 Vierter Bericht d. Komm. z. wiss. Untersuch, d. deutschen Meere. 1884. 



