of the Fishery Board for Scotland, 



125 



V.— ZONES OF GROWTH IN THE SKELETAL STRUCTURES 

 OF GADID.E AND PLEURONEC TID/E. By J. T. Cunning- 

 ham, M.A., F.Z.S. (Plates VII-IX.) 



Previous Investigations, 



The primary object of Reibisch's investigations was to ascertain what 

 relations existed between the number of eggs produced by a plaice and its 

 size or age, whether if the number of eggs varied, it depended on the size 

 or on the age of the fish or on both. In describing his method of enume- 

 rating the eggs to be shed in the following spawning season, Reibisch 

 shows that he was not acquainted with my own paper on the develop- 

 ment of the ovarian egg in Teleostei in general and Pleuronectidse in 

 particular published in the Quarterly Journal of Microscopical Science in 

 1897. For he explains the opacity of the larger eggs in the ovary in 

 August as due to oil-drops — "durch die Aufnahme einer grossen Zahl 

 kleiner fett tropfchen zu erklaren" — whereas I have shown that in 

 Pleuronectes ova there are no oil-drops, but only yolk granules, while in 

 the developing eggs of sole, mackerel, &c, both yolk granules and oil- 

 drops are present and are easily distinguished from one another. 



Reibisch found great variations in the number of ripening eggs in 

 plaice, and these numbers could not be brought into correspondence with 

 either the weight or the length of the fish. He then found that the 

 various numbers formed three principal groups, between which few or no 

 numbers were found : thus there were large numbers of fish with eggs 

 from 50,000 to 170,000, or from 220,000 to 270,000, but scarcely any 

 fish whose number of eggs lay between 170,000 and 220,000. It seemed 

 therefore probable that the groups of numbers corresponded to different 

 ages, and Reibisch sought for a method of ascertaining the age of the fish. 



He rejects entirely the markings of the scales as indications of the age 

 in the plaice, stating that the lamination of the scale can be used for the 

 purpose in view in the carp, but that this is impossible in the case of the 

 plaice. The reasons he gives are that the presence of an annual lami- 

 nation (Jahresschichtung) is scarcely to be demonstrated in the simple 

 cycloid scales of the plaice, and further, that in almost all regions of the 

 latter there occurs a transformation of the cycloid to the ctenoid form. 

 But he seems to have misunderstood Hoffbauer's work on the carp, for 

 that author deduces the age, not from the lamella?, if such exist, but from 

 the varying distance between the concentric lines of the scale, and these 

 also occur in the scales of plaice. I have shown by my observations, 

 described below, that the distinction of the growth of successive years in 

 the scales of the plaice, from the different intervals between the concentric 

 lines, is not impossible. The remarks of Reibisch concerning the 

 transformation of the scales into the ctenoid form in the plaice refers 

 to the Baltic variety on which he worked, in which spinules on the scales 

 are strongly developed, especially in adult males. But this does not affect 

 the anterior embedded part of the scale, and I have not noticed spinules 

 on the scales I have examined. The spinules are developed in adult males 

 in the North Sea, but they are usually confined to limited portions of the 

 i 



