LIFE HISTORY OF DESMOGNATHUS FUSCA. 289 



changes, which take place during the brief terrestrial period, 

 involve no mitosis, not even in the rapidly growing tail and tail 

 fin. They consist, rather, in a readjustment and rearrangement 

 of the cells to suit the changes in the shape and proportions of 

 the body, the full quota of cells having been already formed in 

 the mitotic stage preceding hatching. Thus over the growing 

 tail fin the epidermal cells become stretched apart, the cells of 

 the deeper layer alternating with those of the external layer and 

 in some cases actually reaching the external surface. In the 

 ventral region of the body where the area of the surface is rapidly 

 diminishing with the reduction of the yolk, the cells lose their 

 flattened form and become subspherical or even columnar. 



Over all parts of the body \vith limited exceptions, there is a 

 slight increase in the thickness of the skin from various causes. 

 The cuticular layer of the external cells increases slightly in 

 thickness, probably, however, at the expense of the rest of the 

 cell; the dense corium underlying the epidermis becomes a little 

 thicker; the chief increase comes, however, from that change in 

 the inner layer of epidermal cells which is the most conspicuous 

 change in the skin during the terrestrial larval life, the distension 

 of the cells as if from a condition of turgor. This distended 

 condition soon becomes more marked in certain cells than in 

 others, and these eventually form the greatly enlarged, vacuo- 

 lated cells of the type described by Leydig ('53) and subsequently 

 designated the "Leydig cells" of the epidermis (Ic). These show 

 a somewhat reduced nucleus and a large cytoplasmic region in 

 which a loose reticulum sometimes appears, but which with most 

 stains gives the effect of a clear area between the nucleus and 

 the thin cell membrane. This enlargement of certain cells 

 naturally increases the thickness of the deeper epidermal layer. 

 The Leydig cells press inward upon the corium and outward upon 

 the cells of the external layer; they seem even to push around 

 and between these latter cells but never to actually reach the 

 external surface. The remaining cells of the deeper layer, which 

 lie between the Leydig cells, become crowded together into a 

 columnar form and reach in most cases from the corium to the 

 outer layer of cells. The differentiation of the Leydig cells has 

 already begun at the time of hatching and is most conspicuous 



