of the Fishery Board for Scotland. 



13 



* 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 has been determined is that the 

 ' linal 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, secreted 

 ' by the follicle, which dissolves the yolk-spherules, is associated 

 ' with the dissolution of the germinal vesicle, and the definite for- 

 ' mation of the germinal layer ; 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 den- 

 ' sity — in other words, to become pelagic. The fact has also been 

 ' established 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 more rapid physical or physico- 

 ' chemical change, the fundamental function of which is probably 

 ' the partial solution of the yolk, the dissolution of the germinal 

 ' vesicle, and the rearrangement of the chromatin — the phase mark- 

 ' ing the completion of ovarian growth and the preparation of the 

 ' egg for fertilisation — but which has, so to speak, been specially 

 ' developed for another purpose — namely, to enable the eggs to 

 ' float and to become widely dispersed. 



' The discovery of 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 circula- 

 ' tion, the simple mode by which the yolk is absorbed, the rapidity 



* with which it is absorbed in the larval stage, and the relatively small 



* and ill-developed condition of the larva. It is obvious that, since 

 ' the substance of the embryo and larva is almost entirely derived 

 ' from the yolk, an egg in which the yolk has been diluted with three 

 ' or four times its bulk of watery fluid has only a third or a fourth 



* of the nutritive power of one in which the yolk remains in its 



* original state ; and that, other things being equal, the physiological 

 ' transformation of yolk into embryo ought, in such a case, to be both 



* more rapid and easy, and the resulting embryo either smaller or less 

 ' developed or both. In this sense, so far as concerns nutritive value, 

 ' the mature demersal egg is comparable, not to the mature pelagic 

 ' egg, but rather to the preceding opaque stage ; although the initial 



* nutritive value of the latter, unit for unit, is in reality inferior. 



' A knowledge of the nature of the yolk in pelagic eggs likewise 

 ' explains the gradual sinking of the larvae after they are hatched, 

 ' and even in some cases of the egg containing the advanced embryo. 



* It is in virtue of the watery yolk of low specific gravity that the 

 ' egg floats, and as this becomes used up in the growth of the little 



* fish, to which it is attached, and transformed into its denser tissues, 



* the specific gravity of the whole is increased, until it exceeds that 



