AS AN INDEX OF AGE, 31 



all ctenoid scales are cycloid at the beginning of their formation. Growth 

 does not take place equally in all scales of the same fish, as one may observe 

 scales of different sizes in different parts of the body. Although scales, as a 

 rule, form themselves by the successive addition of new layers to their 

 internal surface, there are some scales, such as those of the tunny and 

 Dadylnptenis volifans, which present difficulties. These scales show inter- 

 nally a spongy tissue, hollowed out into lacun?e of varying size. The 

 structure of these scales affords, according to Baudelot, a connecting liidc 

 between the tissue of scales and osseus tissue with internal lacuna3, such 

 as one observes in the opercular skeleton of various fishes (Gasterosteus). 



In the third part of his monograph, Baudelot takes up the question of 

 *' scales considered from the point of view of classification." He considers 

 this question in relation to the following points: — (1) Connection of scales 

 Avith the integument. (2) The form of scales. (3) The dimensions of scales. 

 (4) The presence or absence of scales. (5) The ridges on scales. (6) The 

 spines on scales. (7) The grooves on scales. (8) The perforating canaliculi 

 and interior lacuniB of scales. (9) The focus of scales. (10) The tissue of 

 scales. In summarising the results derived from a consideration of these 

 points, he concludes that none of these characters of scales taken by them- 

 selves can serve as a basis for the classification of fishes, that the most im- 

 portant of all of them, the cycloid and ctenoid character, does not possess the 

 degree of importance which many zoologists have attached to it, and that the 

 other characters noticed are of still less value. Although each character by 

 itself is of little value, yet the characters of scales as a Avhole ought not to be 

 neglected in establishing natural groups. He recognises that in order to put 

 such a programme into execution a much more precise knowledge is necessary 

 of the external characters, structure, and mode of development of scales in 

 a large number of types of fishes. In this connection he refers to Steeg's 

 paper as a useful essay on scales from the point of view of classification. * 



The next paper which I must notice is that by Ryder on the mechanical 

 genesis of the scales of fishes. f He says in his introduction "that fourteen 

 years previously he had suggested that the slow metamorphosis of the forms of 

 the crowns of the teeth in man, in the course of a vast number of successive 

 generations, might be ascribed to the continuous, slow, and cumulative action 

 of mechanical strains and pressures in definite directions, resulting in the pro- 

 duction of permanent stresses and consequent changes in the forms of the 

 crowns, especially of the molar series. . . . The present paper is an attempt to 

 apply somewhat analogous reasoning to a somewhat simpler, but no less 

 interesting problem in morphogenesis." Scales take their origin from a con- 

 tinuous subepidermal matrix, a basement membrane. This basement mem- 

 brane is thickest on the dorsal and lateral aspects of the body, as seen in 

 sections of the young, for example, in Batraclms tau, a scaleless form. It is 

 "seen in larval stages of scale bearing forms, and may be continuous with a 

 very thin basal membrane from which the primordial fin-rays of embryo fishes 

 seem to be partly differentiated. . . . Such a matrix appears to be co-extensive 

 with the entire epidermic layer of the young in many types of fishes, just at 

 the time when the scale commences to be developed." 



* Steeg, 1857. t Ryder, 1892. 



