AS AN INDEX OF AGE. 35 



little consistency. Within this lies a cellular layer resembling epithelium. 

 This epithelial layer is that described by Hatschek as the "bordering epithe- 

 lium of the dermis." 



At the stage in which the first foundation of the scale appears the skin is 

 about '03 mm. thick. Of that thickness the epidermis occupies about one-half, 

 and consists of four to five layers of cells, of which the most external layer is 

 somewhat flattened. In the remaining part of the epidermis the cells are 

 somewhat cubical and show the presence of nuclei. Mucus cells with sickel- 

 shaped compressed nuclei are also seen, and a thin basal membrane separates 

 the epidermis from the dermis. The outer surface of the epidermis is smooth. 



The dermis consists of a small number of lamellse lying horizontally upon one 

 another. So long as there is no trace of scales, the lamellae in the dermis 

 extend nearly to the epidermis. The dermis cells, which as in earlier stages 

 lie in small numbers between its lamelliB, show somewhat flattened nuclei. 

 The cells become rather more numerous immediately beneath the basal 

 membrane of the epidermis, and the nuclei here are slightly more circular in 

 form than those of the other dermis cells. Chromatophores are also seen at 

 this part ; but of blood-vessels there is no trace in the dermis. Chromato- 

 phores are also to be seen situated above the bordering epithelium of the 

 dermis. Internally to this last follows the musculature, the most external 

 portion of which is made up of only young stages of muscle-fibres. The 

 first foundation of the scale appears as an aggregate of dermis cells lying 

 beneath the basal membrane of the epidermis ; but neither the basal 

 membrane nor the epidermis itself takes any part in the formation of the 

 .scale. The cells which gather together to form the scale foundation are 

 distinguished from the other cells of the dermis in possessing larger nuclei 

 and a better-developed protoplasmic body. This cell-mass, the foundation 

 of the scale, resembles epithelial tissue. Each scale germ presses upward 

 on the basal membrane of the epidermis as a slightly arched papilla. 

 During this upward growth of the scale germ the upper surface of the 

 epidermis remains smooth ; but at the places where a scale germ is situated 

 the epidermis is reduced from five to two or three layers of cells. In trans- 

 verse section the scale germs are seen as papillse, Avhose highest points 

 are not exactly at the centres of the masses of cells, but are situated 

 slightly caudalwards. These cell-masses (scale germs), which approach the 

 circular form in surface view, stand free from one another in regular rows, 

 diagonal to the body axis. Later the entire cell-mass spreads itself out hori- 

 zontally, and its elements arrange themselves in two slightly flattened layers. 

 Between those two layers there appears a thin layer of stroncjhj refractive 

 suhstance. In transverse section it is seen that the formative cells lay down 

 the new substance, alternately on the outside and on the inside, producing 

 what looks like a slightly undulated plate. The form of the plate is approxi- 

 mately circular, corresponding to the form of the cell-mass. These plates can 

 be isolated and represent small scales. The strongly refractive substance later 

 on shows itself to be the hard substance of the scale : at what period this 

 plate impregnates itself with lime salts Klaatsch has not investigated. The 

 formative cells which give rise to the scale are known as srh'rohla.-ifr^, and 

 they correspond to similar elements in Selachians and Ganoids. At this 



