288 INEZ WHIPPLE WILDER. 



type, the body of the cell often lying between the two layers of 

 epidermal cells. These branches seem particularly to embrace 

 the cells of the external layer, though occurring between the cells 

 of the deeper layer also. Finally, beneath the corium and 

 closely applied to it, is an interrupted layer of enormous patches 

 of pigment matter which, however, does not seem to penetrate 

 the corium and encroach upon the epidermis. In the unpig- 

 mented ventral region all of these forms of pigmentation are 

 lacking. 



In comparison with the skin of the 13 mm. embryo the skin of 

 the newly hatched larva of 15.5 mm. impresses one as a fully 

 formed structure (Plate IV., 17 and 18). In many regions of the 

 13 mm. embryo the epidermis is very thin, and although two 

 layers of cells are present, the nuclei, particularly of the outer 

 layer, are often so widely separated, owing to the flattened form 

 of the cells, as to seem in a vertical section through the skin to 

 be entirely lacking for considerable distances (cf. Fig. n). 

 There is only the merest trace of cuticular structure in the 

 external border of the outer cells, and practically no corium 

 beneath the lower cells. Branched pigment cells occur both 

 beneath and between the epidermal layers but there is no evi- 

 dence whatever of intracellular pigmentation of the epidermis. 

 The embryonal character of the epidermis is evidenced by the 

 slightly rounded external contour of the outer layer of cells, and 

 by the large size of the nuclei, which, moreover, present an 

 enormous amount of mitosis (ml) in both layers. As might be 

 expected from the fact that the larval two-layered condition has 

 already been reached, the mitotic figures are oriented with their 

 axes parallel with the external surface of the body, and the process 

 is, of course, giving rise to an increase in the number of cells in 

 each layer. This condition of general mitosis contrasts sharply 

 with the limited amount of mitosis in progress in the epidermis 

 of the newly hatched larva, where the figures are, moreover, in 

 late anaphase or telophase. 



There occur after hatching but few changes in the skin of the 

 larva to complete this rapid transformation from the embryonal 

 condition to that which is to serve the animal during its eight or 

 more months of larval life (Plate IV., 23, 24, and 25). These 



