of the Fishery ^oard for ScoUand. 



80 



important paper on the development of osseous fishes, dealing principally 

 with marine forms (Gasterosteas spinachia, Gobius minutus, G. niger)* 

 In 1874 Hackelt and E. Van BenedenJ studied the development of 

 unidentified pelagic fish-eggs in the Mediterranean ; and from about that 

 time, and especially during the last ten or^ fifteen years, a large number of 

 memoirs and papers dealing with the eggs and the embryology of teleostean 

 fishes have been published in this country, principally by Professor 

 M'Intosh, Mr J. T. Cunningham, and Mr E. W. L. Holt. 



The researches that have been made on the eggs and embryology of 

 marine Teleosteans since the discoveries of Sars, while of great value from 

 the scientific point of view, have been also of direct practical importance 

 in showing that the eggs are in the great majority of species buoyant or 

 pelagic, and that those which are demersal are, for the most part, shore 

 forms, which, with the exception of the herring, are of little economic 

 utility. All the Gadoids (except Lota vulgaris, which is a fresh-water 

 form) and the European flat-fishes have pelagic eggs, as have also the 

 mackerel, pilchard, anchovy, sprat, the gurnards, and many others. On 

 the other hand, none of the species which spawn in fresh water or in 

 estuaries have pelagic eggs, for a reason which will be apparent later. 



The investigations described in this paper deal mainly with the later 

 stages in the ovarian development of the teleostean ovum, especially the 

 maturation of the pelagic ovum, and with certain phenomena which arise 

 in consequence of the changes which occur. 



The most important point which I have been able to determine 

 is that the final change in the maturation of the pelagic ovum, 

 while still within the ovary, is accompanied by a comparatively 

 rapid and relatively great accession of a watery fluid, of low density, 

 from without, which dissolves the yolk-spherules, is associated with 

 the dissolution of the germinal vesicle, and the definite formation 

 of the jDeriblast, distends the ovum to three or four times its former 

 volume, thinning the capsule correspondingly, renders it of crystalline 

 transparency, and reduces its specific gravity so that it is enabled to float 

 in sea-water of ordinary density — in other words, to become pelagic. I 

 have also been able to establish the fact that this change likewise occurs 

 in the demersal eggs of Teleosteans at maturation, but to a less extent, the 

 quantity of fluid absorbed being much smaller in amount, and the yolk- 

 spherules remaining comparatively little affected. 



The process is different from the ordinary slow growth of the eggs 

 within the follicle. It is a rapid physical or physico-chemical change 

 which is associated with the dissolution of the germinal vesicle and 

 the rearrangement of the chromatin for fertilisation — the phase marking 

 the completion of ovarian growth and the preparation of the egg for 

 fertilisation — but which has, so to speak, in the pelagic egg, been seized 

 upon and exaggerated by natural selection for another purpose — namely, 

 to enable the eggs to float and to become widely dispersed. 



The solution of the spherules and the dilution of the nutritive yolk by 

 several times its volume of water, explains a number of phenomena 

 hitherto obscure in the development of the embryo of pelagic eggs, such 

 as its comparatively brief duration, the absence of true vitelline circulation, 

 the simple mode by which the yolk is absorbed, the rapidity with which it 



* *' Beobachtnngen liber d. Entwickelung d. Knochenfische." Arch, f. Mikr. Anat., 

 Bd. iiK, p. 209. 1868. 



t Die Gastrula und die Eifurchung der Thiere. Jena Zcitschr> ix., 1875. 



X A contribution to the History of Embryonic Development of Teleosteans. 

 Quart. Journ. Micr. Set. xviii., 1878. 



