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Part III. — Sixteenth Annual Beport 



** the exterior. The difference between the unripe and the ripe egg is due 

 " to a change in the condition of the yolk, which becomes more liquid and 



less granular."* The same naturalist, in a more recent memoir dealing 

 with the ovarian eggs of Teleosteans, speaking of the large opaque ovarian 

 eggs of the gurnard, says — "Eggs which are larger than this begin to 

 " grow transparent again. ... As the egg approaches the ripe condi- 

 " tion both the oil-globules and the vitelline begin to fuse together, . . . 

 " the vitelline globules in fusing form a continuous liquid, in which the still 

 " unfused globules remain suspended." In describing the ovarian eggs of 

 the turbot he points to the fact that there are scarcely any intermediate 

 stages between the opaque eggs and the mature transparent eggs, " but in 



the nearly ripe condition the protoplasmic strands separating the yolk- 

 " globules are not visible towards the centre of the egg. It is evident, 



therefore, that the fusion of the yolk-spherules takes place almost 

 " simultaneously. . . . that the yolk swells considerably during the 

 " final stage of maturation."t Further, M'Intosh and Masterman, in their 

 recent work on the life-histories of marine fishes, describe the change as a 

 " clearing up of the yolk mass before ripening," that " towards maturity 

 " the granules of the yolk disappear and the ovarian egg becomes quite 

 " translucent."^ 



About seven years ago, when making an investigation on the fecundity 

 of marine fishes, and consequently examining a number of ovaries, I was 

 much impressed with the striking change that takes place at maturation 

 in pelagic eggs ; and I then described the " more or less sudden increase 

 in bulk that occurs in the ovum shortly prior to its extrusion," and stated 

 that it seemed " to be due to sudden accession of fluid from the ovarian 

 " follicles, which increases the bulk of the ovum and renders the opaque 

 contents clear by dilution." I also stated that it was at this stage of 

 " hyaline enlargement " that pelagic ova first acquired the property of 

 buoyancy ; that a similar but less marked change occurred in certain 

 demersal ova, and that " the[process is probably essentially similar both in 

 " pelagic and such demersal ova." I was, however, in error, I believe, in 

 supposing that the change does not occur in all demersal eggs ; further 

 research will probably show that it is common to all. In the same paper 

 I stated that the degree of the increase of bulk varied in different species, 

 and showed that the translucent ripe eggs of the plaice were about four 

 times larger, and about four times heavier, than the large opaque eggs 

 just before the change took place. § 



In describing the changes which occur at maturation I shall first of all 

 deal with the prominent physical features and properties. 



The most striking change is the transformation in the appearance of 

 the ovum from an opaque sphere, chalky white by reflected light, to a 

 clear globe of crystalline transparency, looking like a droplet of dew. 



Another change, perhaps less obvious to the eye, is even more 

 significant, namely the great increase in volume which occurs. From 

 the fact that the eggs are spherical (although not always perfect spheres) 

 a slight increase in the diameter may not attract much attention, although 

 it may mean a considerable increase in the volume. The formula for the 

 contents of a sphere is = ^ x d^ ; and applying this to the eggs at the 

 opaque stage and at the mature translucent stage it will be found that 

 there is a very great difference in the volume in the two cases. For 



* The Natural History of the Marketable Marine fishes of the British Islands, pp. 

 71,97. 1896. 



t Quart. Joum. Micr. Sci. Vol. xl. Parti., pp. 102, 110. 

 + The Life-Histories of the British Marine Fooff- Fishes, pp. 18, 72. 1897. 

 § The Comparative Fecundity of Sea Fishes, Ninth Anmial Report Fishery Board for 

 Scotland Jar the year 1890. Part Hi , p. 243. 1891. 



