3IO INEZ WHIFFLE WILDER. 



8. The whole external surface gives evidence of the formation 

 of a thin moult layer of epidermis, which here and there becomes 

 separated from the deeper layers and thus presents a loose 

 appearance. 



In individuals in captivity these changes in external appearance 

 have been observed to occur in the course of a few days during 

 which time the individual crawls persistently out of the water. 

 The metamorphosis may be considered quite complete when, in 

 addition to the above enumerated changes in external appearance, 

 the distinctive physiological metamorphic phenomenon occurs, 

 and the animal, with mouth tightly closed, lowers the floor of 

 the mouth and pharynx, and, drawing in air through the short 

 nasal passages, fills its bucco-pharyngeal cavity and thus estab- 

 lishes aerial respiration. 



Although the final steps in metamorphosis take place with 

 great rapidity, the histological changes preliminary to the 

 process begin considerably earlier. Thus specimens of larvae 

 collected in May show decided indications of the approaching 

 change. The most striking of these indications is the large 

 amount of mitosis which is in progress especially in the deeper 

 layer of the epidermis (Plate V., 26, 27 and 29), resulting in so 

 rapid a multiplication of the epidermal cells that the earlier 

 arrangement in two fairly definite layers gives place to a con- 

 siderable irregularity of arrangement of the cells of the deeper 

 layer, the epidermis thus coming practically to consist of three 

 layers. The arrangement of the cells of the outer layer remains 

 unaltered, however, their cuticular borders forming, as before, 

 a continuous external covering. The cells of the outer layer 

 become, however, more and more flattened and are eventually 

 to be cast off as the first moult layer, not, however, until after 

 metamorphosis. 



This flattening of the external layer of cells in part accounts 

 for the actual decrease in thickness of the epidermis which 

 becomes evident as the time of metamorphosis approaches; by 

 far the greater cause of this, however, lies in the gradual dis- 

 appearance of the Leydig cells, which by their turgor so greatly 

 increase the thickness of the epidermis during early larval life. 

 The invasion of these cells by pigment cells, connective tissue 



