LIFE HISTORY OF DESMOGNATHUS FUSCA. 3OI 



which at times renders them the most conspicuous portions of 

 the body. This appearance is similar to certain white spots which 

 occur along the lateral surfaces of the body and occasionally in 

 other regions, in both the larval and the adult stages, and have 

 been described as "white pigment spots." There is considerable 

 evidence, however, that in the gills, at least, the phenomenon is 

 due to an accumulation of some gas, presumably carbon dioxide, 

 in the tissues, but its exact nature and significance demand 

 further investigation. 



In the matter of internal structure as in size, the aquatic larva 

 appears to be in a general static condition throughout the larger 

 part of its larval life. Late in the spring, however, shortly before 

 the metamorphosis, a series of changes becomes inaugurated, 

 involving especially the integument and the organs derived 

 from it, as well as certain skeletal parts, and leading rapidly 

 to the conditions which characterize the adult. Such changes 

 are to be considered as belonging particularly to the period of 

 metamorphosis, and will be described as such. 



Until such distinctive metamorphic changes appear, the skin 

 of the larva remains in practically the condition attained at the 

 end of the terrestrial period. The two layers of cells of the 

 epidermis remain fairly well defined. The cuticular border of 

 the outer layer becomes thicker and more compact in appearance, 

 and the cap of intracellular pigment between this border and the 

 nucleus becomes denser. In the deeper layer, the cells of which 

 rest directly upon the dense corium, the distended vacuolated 

 Leydig cells are in most regions so abundant that each is usually 

 separated from the neighboring Leydig cells by but a single 

 circle of cells of the general epidermal type (Plate IV., 25). 

 Here and there among the cells of the deeper layer are cells which 

 are considerably elongated and somewhat piriform and which 

 extend out between the cells of the outer layer almost to the 

 external surface. These are the cells which in the German 

 literature are designated as the "Schaltzelle" (Plate IV., 23, 

 intr). As the larval period advances there is a gradual increase 

 in the thickness of the epidermis due to the increasing size of the 

 cells of the deeper layer, especially the Leydig cells, which grad- 

 ually become more distended, while their nuclei become notice- 



