AS AN INDEX OF AGE. 43 



out in all, which Ilatschek termed the ''marginal epitlieliiun of the cutis." 

 According to some authors, the dermis is separated from the epidermis by a 

 thin membrane, a distinct and independent structure, sometimes termed the 

 ground membrane. Toldt, in his "Lehrhuch der GewebeJehre," however, says, 

 " It has now been almost generally accepted, not as an independent structure, 

 but as a modification and thickening of the upper layer of the connective 

 tissue ground substance." Ussow thinks that this membrane as an independent 

 structure does not occur in the families Cobitidre and CyprinidiE, but simply 

 that a transition substance of connective tissue devoid of fibres lies between 

 the epidermis and the dermis. 



The first stage in the development of a scale consists of fairly distinct and 

 prominent aggregations of mesoderm elements in the upper half of the dermis, 

 immediately beneath -the epidermis. The cells forming such a papilla, as we 

 may call these aggregations, differ at least in the beginning from the other 

 cells of the dermis, and no ground substance is developed between them. 

 This papilla gradually grows out in a horizontal direction, pushing the epi- 

 dermis before it slightly upwards. When the papilla has reached a certain 

 stage, a change takes place in its constituent cells. All cells excepting the 

 lower become more circular in form and their nuclei gradually become more 

 transparent ; the lower cells, on the contrary, are, as before, highly coloured and 

 their nuclei are spindle-shaped. In the next stage a separation of the elements 

 of the jyapilla into two layers, an upper and under, becomes distinctly observed. 

 Between these two layers a thin strip of highly refractive substance stands out 

 p)rominently. At the commencement this strip does not extend throughout 

 the entire length of the papilla, and one may see, in sections, that it is 

 thickest at the centre, and gradually thins out towards its border, until at the 

 end of the section of the papilla the strip is not visible. 



The secretion of this refractive substance thus does not commence with the 

 peripheral elements, but with the cells found towards the centre of the papilla. 

 In further develoj)ment, the substance of the first layer of the scale shows itself 

 throughout the entire length of the section of the papilla, and the strips also 

 become broader ; meanwhile the papilla grows out in a horizontal direction. 

 In this way a round curved plate originates, lying parallel to the upper surface 

 of the body of the fish close beneath the epidermis. The upper and lower 

 surfaces of this plate are formed out of scleroblast cells (the formative cells of 

 the scales). The upper layer of scleroblasts simulates in its appearance a flat 

 epithelium with clear spaces between its component cells. Later, each of the 

 constituent cells changes, its nucleus comes to lie towards one end, and a 

 circular colourless space appears at the opposite end. Klaatsch held that the 

 clear spots within the cells fuse with the clear spaces between the cells ; but 

 Ussow did not observe any such fusion in his preparations. Klaatsch'a 

 description of these processes does not appear at all clear to Ussow. Klaatscli 

 says that the cell structure on the external surface of the scale sliows diiferen- 

 tiation into several layers of cells, and that these cells build up the substance 

 of the first layer of the scales. The question would naturally arise, how it is 

 that the cells of the lower row do not become covered by the product of the 

 upper cells. It would seem that the lower cells would become quite changed 

 by the product secreted on them ; but according to Klaatsch tliis docs not 



