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Part III. — Sixteenth Annual Report 



is a\)sorbed 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 Huid has only a third or a fourth of the nutritive power of one in 

 \vhich 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, as 

 will be shown later, 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 of the sea water in which it is immersed. 

 Hence the general rule that pelagic eggs are obtained in the surface 

 layers of the sea, while the larvae are found most abundantly towards the 

 bottom and in the middle layers. The expansion of the pelagic ovum at 

 maturation will also be afterwards shown to be correlated in certain 

 important respects with the life-history of the species. 



That the fully mature pelagic ovum of Teleosteans is translucent and 

 larger than the opaque eggs present at the same time in the ovary has 

 long been known, but I have been unable to find in any of the numerous 

 papers dealing with them any indication of the real change which occurs. 

 As will be more fully explained later, the descriptions given by various 

 authors afford no hint that it is due to a special cause. The reason that 

 this stage has hitherto escaped close attention is most probably owing to 

 the circumstance that the few who have concerned themselves with 

 ovarian pelagic eggs have, for the most part, studied only sections of the 

 ovary, showing the immature or opaque ova, while those who have dealt 

 with the fully mature eggs have almost always been concerned with the 

 changes following fertilisation, or with their identification and morpho- 

 logical description ; perhaps also from failure to keep clearly in mind the 

 fact that a small increase in the diameter of a sphere represents a large 

 increase in its volume. 



The methods adopted in my investigations were as follows : — (1), The 

 examination of the immature and ripe ovaries in the living fish or in the 

 fresh state ; (2), the microscopical examination of parts of the fresh 

 ovaries and of the ovarian eggs at various stages, without preparation, in 

 various media ; (3), the study of stained and mounted sections prepared 

 in the usual way ; (4), the determination in certain cases of the general 

 nature and quantity of the intra-ovarian fluid present in ripening ovaries; 

 (5), experimental observations on the physical properties of the opaque 

 and the mature eggs, and on some other points. It became evident 

 to me that an adequate explanation of the chemical processes that 

 occur in the maturation of the eggs could only be obtained by the assist- 

 ance of an expert in physiological chemistry. The Board accordingly 

 appointed Dr. T. H. Milroy, assistant to the Professor of Physiology 

 in the University of Edinburgh, for this purpose. The thesis embodying 

 my results was submitted to Dr. Milroy, whose report will be found at 

 page 135 of this Report. 



