12 THE PERIODIC GROWTH OF SCALES IN GADID.E 



tliat the latter paper is " one of the most important which has been published 

 on the scales of osseous fishes." 



Williamson brought forwanl important general vieAvs relating to the mode 

 of composition of the scales and of the other hard parts in fishes. He en- 

 deavoured to show that scales, teeth, chondrified and membranous bones, 

 etc., are not really formed of tissues of an entirely different nature, but 

 of tissues which pass the one into the other by gradual transitions. AVilliam- 

 son commences his paper by a critical review of Mandl and Agassiz's work. 

 He regarded Mandl's view of scale formation as given on page 7 as being 

 more correct in some respects than that of other writers, but as being built 

 upon a false foundation on account of his having mistaken solid calcareous 

 granules for cells. He regarded Mandl's description of the inferior layer as 

 correct, but denied the existence of longitudinal canals as described by him. 



Williamson points out that although Agassiz at first refuted Mandl's state- 

 ment as to their being two layers in scales, he subsequently acknowledged 

 that each scale really consisted of two different strata. Williamson regarded 

 Agassiz's views to be as little tenable as those of Mandl. He says that while 

 Agassiz regarded the lower layer of the scale "as a horny substance, an 

 exuded secretion from the sac into which he considers the lower and anterior 

 portions of the scale to be fitted," it is really a fibrous substance. 



He says that Agassiz has failed " to detect the existence of two distinct 

 structures in the upper or calcified part of the scale," and that in regarding 

 the corpuscles in the middle of the scale not as true corpuscles, but rather as 

 due to some solution of continuity between the upper and lower tissue, he 

 has quite mistaken their character. According to Williamson, cycloid and 

 ctenoid scales consist of three layers, inferior, median, and superior. 



The inferior layer consists of numerous membranous laminae arranged in 

 parallel horizontal lines. These laminse are most numerous in the centre of 

 the scale, and decrease in number as we approach the periphery, until finally 

 only one is present. Each of these membranous laminae is composed of 

 numerous fine fibres, all of which run parallel with one another in the 

 same lamina. Numerous isolated lenticular calcareous bodies are to be 

 observed imbedded amongst these membranous laminae. These calcareous 

 bodies arise as a result of the calcification of the membranous laminas, and 

 appear firstly as small calcareous atoms, which grow in size by the addition 

 of successive concentric laminae to their external borders. "The growth 

 in size of cycloid and ctenoid scales takes place by the successive addition 

 of membranous lamellae on the inferior face of those which have been 

 previously formed, each new plate being larger than the preceding." 



The median layer of the scale is mainly built up of a mass of similar 

 lenticular calcareous bodies which unite with one another as they increase in 

 size, frequently also losing their original lenticular shape during this process 

 of coalescence. 



This median layer of the scale decreases in thickness as one proceeds from 

 the centre to the periphery until at last it disappears altogether, the calcareous 

 layer being not only thicker, but now consolidated towards the centre of the 

 scale. After the calcareous granules have become fused and consolidated 

 together, the median layer thus formed is split up into horizontal laminae 



