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Part III. — Twenty -third Annual Report 



slender curved rod of bone, and a somewhat quadrate dorsal portion of 

 calcified cartilage. The centre of growth is at the junction of the bony rod 

 with the cartilaginous plate, and at intervals there are lines parallel to one 

 another where calcification is more complete, and the cartilage conse- 

 quently more opaque. The scapula shows similar lines, and they are 

 parallel to the edges where the two elements meet. The basal elements 

 with which the fin rays articulate are represented by a small plate of 

 cartilage with three or four minute points of ossification. 



The faces of the vertebra? show by reflected light opaque white bands 

 separated by darker, more transparent lines, and these are probably 

 annual increments of growth, but it is difficult to be sure of counting the 

 complete number, as the more central ones seem always doubtful and 

 indistinct. 



In the cod the scales (fig. 15) exhibit concentric and radiating lines 

 as in the plaice, but the radiating lines occur all round the scale, and 

 the concentric lines are much more distinct and farther apart. Careful 

 examination shows that the two systems of lines are due to the fact that 

 the outer surface of the scale is made up of rings of separate elements, 

 which may be conveniently termed sclerites. Each sclerite consists of a 

 flat base with a projecting ridge, the ridges being placed in line with 

 those of the neighbouring sclerites of the same ring. The ridge is 

 situated near the outer border of the sclerite, and the edge of it is turned 

 towards the hilum or focus of the scale, so that there is a depression or 

 concavity on the inner side of the ridge. These sclerites are evidently the 

 structures described by Marett Tims as scalelets in the passages quoted 

 from his paper by J. Stuart Thompson {Jour. Mar. Biol. Asm., No. 1, 

 1904). He states, however, that each scalelet consists of a basal plate 

 with a minute spine projecting from its upper surface, a description which 

 he has apparently taken from the appearance presented in transverse 

 section, whereas the apparent spine is merely the section of a longitudinal 

 ridge on the sclerite, as I have stated. The ridges on the sclerites are also 

 identical with the rolls or cylinders which cover the surface of the scale, 

 according to Ussow's description. 



According to Marett Tims, the scalelets are covered with a delicate 

 epidermis. In my sections I find cell-nuclei both on the upper and lower 

 surface of the scale, and have no doubt that the scale is covered with cells, 

 to whose activity its formation is due. These cells or their nuclei are 

 particularly evident at the edges of the scale, where it increases 

 in extent, and where the new sclerites are successively formed. Nuclei 

 can also be seen in the concavity on the inner side of the ridge of 

 the sclerite, which agrees with Klaatsch's statement " that the cells 

 arrange themselves on the surface of the scale in curved rows, and form 

 always in front of themselves the substance out of which the rolls are 

 made." I find, however, that the new sclerites are formed only at the edge, 

 and that the above description of the cells and " rolls" or ridges applies 

 only to the upper surface of the scale. Fig. 17, pi. ix., shows the appear- 

 ance of a transverse section of the skin of a cod under a low power. The 

 epidermis is of considerable thickness, and consists of small cells whose 

 boundaries are not distinct in the preparation, but whose nuclei are seen 

 as dots. The lowest layer of nuclei are somewhat elongated in a direction 

 vertical to the lower surface of the epidermis. Beneath this lower layer 

 are seen large oval masses of black pigment, the sections of the chromo- 

 blasts. In the middle region of the epidermis are a number of rounded 

 cavities. The specimen from which the preparation was made was pre- 

 served with formaline, and I am unable to state whether these cavities 

 exist in the living skin or are the result of the action of the formaline. At 

 the surface of the epidermis are seen two minute somewhat fusiform 



