AS AN INDEX OF AGE. 39 



hardened by the deposit of lime salts, and the described cell layer is simply a 

 layer of scleroblasts, which are only distinguished by regularity of arrange- 

 ment and by sharp marking of individual stages of the scleroblastic processes 

 from the deeper cells of a similar kind with which they are continuously 

 dependent at their borders. The nearer the scleroblasts are towards the 

 margin the more do they show (though here no longer separable into layers) 

 an increase of their cell-body in a tangential and a decrease in the radial 

 direction in relation to the entire scale. As in other Physostomi, ridges are 

 formed on the external surface of the scales of the trout. These ridges have 

 a concentric arrangement on the scales of this fish, which is not, however, 

 a general rule for superficial reliefs. In the trout the cells concerned arrange 

 themselves so that they correspond exactly with the concentric ridges. One 

 might expect that the superficial scleroblast layer would cover the deeper cell 

 layer with its product, so that the constituent parts of the last would be taken 

 up into the interior of the scale substance. This does not, however, take 

 place in the trout. The cells arrange themselves as they pass through the 

 changes described, so that they come to lie on the external surface of the 

 ridges and contribute to the enlargement of these. They elaborate, as it were, 

 the upper relief surface of the scale, for which the deeper cells had only 

 supplied the foundation. 



In the older stages and in the mature condition of all the scleroblasts there 

 remain only the nuclei and small masses of protoplasm. One sees the cells 

 lying on the surface of the scale ; if one takes a scale from a living fish, for 

 example from one of the Cyprinoids, and observes it in a fixing lluid, say 

 chromic acid, then one easily recognises circular nuclei surrounded by proto- 

 plasmic masses which extend in fine continuations. A similar condition to 

 that in the trout appears in other Teleosteans; in many Clupeoids, for example 

 in Elops saurus, Albula conorhijnclms, cells are found perfectly similar in their 

 arrangement to those in the trout and surrounded by scale substance ; these 

 represent true osseous cells adjoining the concentric ridges. In other forms 

 there are numerous osseous cells present in the scales. 



In Osteoglossum, for example, the wealth of bony cells, and in consequence 

 the thickness of the cell-containing layer, is very apparent. In this form a 

 true cell-containing osseous tissue constitutes an essential part of the scale; in 

 the trout scale, so far as it has hitherto been observed, a similar tissue must be 

 recognised. Its scleroblasts are osteoblasts ; whether these become enclosed 

 by their product or not is of subordinate significance, as in related forms some- 

 times the one, sometimes the other is the case. 



Originally the outer and the inner surfaces of the scale are alike in 

 regard to their scleroblasts. On the inner surface, however, the scleroblastic 

 processes gradually take another direction. As the outer and inner sclero- 

 blasts gradually pass into one another at the margin of the scale, and as both 

 originate from the same cell material, no sharp separation can be drawn 

 between them. 



The scales retain for a lengthened period of their ontogeny the structure of 

 a thin bony plate, whose growth takes place especially at the margins and at 

 the external surface. Not until the time when the scales have reached the 

 condition of being a tile-like covering does a considerable increase of volume 



