34 THE PERIODIC GROWin OF SCALES IN GADID^ 



built up of the fibrous processes of the ckep dermis layer. Near the scale its 

 condition changes, as immediately towards the inside of the same, numerous 

 cells are found in a ground substance only slightly developed and not fibril- 

 lated. The fibres of the deep dermis layer have a similar arrangement to that 

 of Ganoids and Selachians. 



One may easily ascertain this by observing a piece of skin from the surface. 

 The fibres of one layer of fibrous bundles cross those of the next higher or 

 deeper layer in such a way that, in relation to the long axis of the fish, the 

 anterior and posterior angles of intersection are greater than right angles. 

 The fibres surround the body in a diagonal direction to the body axis, corre- 

 sponding to the rows of scales. Towards the musculature the dermis is 

 bordered by a layer of cells which resemble the other cells of the dermis, but 

 lie closer to one another. In this part chromatophores are also seen. Under- 

 neath the dermis the musculature only shows young fibres similar to those seen 

 in immature forms, xis regards number, the scales have nothing at all to do 

 with the myocommata. Several scales are usually found on a myocomma; 

 the relation to metamery suggested by Salbey does not exist. 



The development of the teleostean scale has hitherto not been worked out ; 

 one only finds a few incomplete references to this subject. The first who 

 takes any notice of the subject is C. Yogt, in his " Embryologie des 

 Salmones," who mentions " poches epitheliennes," in which the scales are 

 formed. According to him, these pockets are simply folds of the epidermal 

 membrane. This point will be referred to further on. 



Later Leydig devoted some attention to the structure of scales, but did 

 not concern himself with their ontogeny. He says, " The scales of most of our 

 fresh-water fishes appear partly as ossifications of flattened skin continuations 

 which one generally terms scale pockets." He regards scales as fusion products, 

 " peculiarly developed calcareous globules, concretions, or scale corpuscles," such 

 as one finds on the lower side of scales in many Teleosteans. 



Baudelot held the same view as that of Leydig. Although Baudelot's work 

 appeared in 1873, he does not make any note as to the part which cells take 

 in scale formation; "according to him, the scale is simply a conglomerate of 

 calcareous concretions or scale corpuscles, with whose measurements he fills 

 many pages of an extensive treatise." 



Development of scales in the trout. Klaatsch followed the development of 

 cycloid scales mainly in the trout ; but he also made use of Esox and several 

 Cyprinoids for some of the earliest stages. The following are the results of his 

 investigations : — 



In the trout the first formation of scales appears several months after 

 hatching. Trout 2 cm. in length show no scales, but somewhat older ones show 

 the commencement of scale formation. Scales firstly originate in the anterior 

 and median region of the trunk near the lateral line, and their formation 

 extends from this region caudal wards, as well as ventrally and dorsally. 



For this reason trout 3 cm. in length are suitable specimens for the study of 

 scale formation, since older and younger stages occur near one another, the 

 younger being more posterior. Before scale formation commences, the skin 

 of a trout shows a thin epidermis and relatively very fine dermis. In 

 the just hatched trout, the dermis is represented as a homogeneous layer of 



