AS AN INDEX OF AGE, 37 



consequence of this change of position is that the scale, not being hindered by- 

 adjoining structures, can increase the extent of its surface in an oblique 

 direction. A necessary result of further growth is that scales push themselves 

 under adjoining anterior scales by their anterior borders, so that they begin 

 to cover one another like tiles. In order to understand further changes 

 it is necessary to bear in mind that all layers of the skin increase con- 

 tinuously in thickness. The deep lamellar layer of the dermis takes, in anti- 

 thesis to early stages, a stronger growth, and in this development it is the 

 outer layer next the scales which undergoes a change. The epidermis also 

 grows, as well as the continuations of the same underneath the posterior 

 part of each scale. In this inward growth of the epidermis no tissue change 

 takes place ; for instance, one finds in these continuations similar mucus 

 cells to those in the rest of the epidermis. This growth of the epidermic 

 continuations is not to be regarded as a process proceeding from the upper skin 

 alone, but as the result of growth taking place in the entire skin. In this 

 connection the constant increase of the scale at its posterior border is of 

 significance. The anterior border of the scale inserts itself always deeper in 

 the loose connective tissue of the dermis, whose stronger development towards 

 the upper half of the deep dermis has already been noticed. It therefore 

 happens that the scale does not lie next to the deep dermis ; but it gives rise 

 to an appearance as if the scales had pushed themselves between the lamellae. 

 This takes place because the loose connective tissue underneath the scale 

 gradually becomes diflerentiated in a similar manner to that which had taken 

 place earlier in the deeper part of the dermis, and in this case also lamellar 

 fibrillated bundles are formed. These lamellae do not, however, lie parallel 

 to the surface of the body, but parallel to the scale. The lamellae form 

 themselves in the same manner as the scales, growing stronger towards 

 tlie anterior part; the dermis layer situated between the scales becomes so 

 arranged that connective tissue septa exist between the scales. These septa, 

 whicli are the inner walls of the scale pocket, are connected externally with 

 the epidermic continuations, and internally they grade imperceptibly into the 

 deep layer of the dermis. By the foregoing means the scale pockets come into 

 complete formation. These scale pockets appear consequently as a result of 

 scale growth. In this two different processes operate together: on the one 

 hand the scale becomes separated from the epidermis by growing connective 

 tissue, and so an outer wall to the pocket is formed in its anterior part; on 

 the other hand the floor of the pocket and the posterior part of its outer wall 

 is formed by the ingrowth of the scale into the loose portion of the dermis 

 and by the development of the same. The floor of the scale pocket is of 

 special significance in the development of the scale. The tissue of the dermis 

 which produced the floor of the scale pocket retains immediately underneath 

 the scale its indifferent state. Here there lie cells in a ground substance 

 Avhich is not yet broken up into fibrillae. When the same prove themselves 

 active as scale formers, they lead to the formation of a deep scale layer, which 

 shows in its histological relationship much peculiarity. A superficial view of 

 the latest stages shows how the scales gradually insert themselves underneath 

 the three next anterior, until we arrive at tlie condition found in older fish. 

 The median point of the scale becomes distinctly prominent by the formation 



