302 CELLS, TISSUES, AND ORGANISMS 



epithelial cells. The epidermis may digest and utilize some of the 

 products; the remainder may gradually be moved to the surface, with 

 the continuous movement and loss of epidermal cells. 



Normal epidermis may also function, albeit in a less highly active 

 way, in excretion and digestion of debris. A circulation of substances 

 from below into epithelium must exist to satisfy the metabolic needs 

 of the living epidermis. There is evidence in the literature on higher 

 forms that fluid moves through the epidermis from the inside to the out, 

 although the movement is slight ( Rothman, 1954 ) , and it may be that 

 particles of all sorts are caught within this flow to the epidermis. Em- 

 bryonic epithelium is also worth mentioning in passing at this point. 

 The general similarity between embryonic development and regenera- 

 tion of the amphibian limb suggests that embryonic epithelium, like 

 the regenerating one, may also be phagocytic and function to dispose 

 of cellular breakdown products that accompany development. Indeed 

 Ide-Rozas ( 1936 ) has shown that heated yolk granules injected into a 

 young Axolotl embryo are phagocytised by ectodermal cells. 



Although our attention has focused upon a distal direction of 

 movement of substances through the epithelium, it is quite possible 

 that the reverse movement occurs at the same time for certain classes 

 of substances of molecular and colloidal dimensions. In some experi- 

 ments in our laboratory we have observed that relatively large mole- 

 cules, such as colchicine, do penetrate into the blastema by passage 

 through the epithelium when the stump alone is soaked in a solution 

 of that substance. Movement of substances through normal epidermis 

 to deeper parts is well known in the mammal (reviewed by Rothman, 

 1954); it is called "percutaneous absorption." Vitamins, fats, hormones, 

 and ions of various sorts move inward when applied to the epidermal 

 surface. It may be that in some way percutaneous absorption and distal 

 flow operate at the same time. 



The epidermal function of collecting and disposing of substances 

 of the blastema is not reserved for small particles and individual cells. 

 We have seen large bodies drawn into the epithelium by massive as- 

 sociated movements of epidermal cells. Thus the epidermal response 

 is adjusted to particle size, in one case by gross movements of the 

 epithelial sheet to surround the large particle, in the other case by im- 

 perceptible changes in shape to collect smaller particles. The efiFective 

 stimulus that excites the response may be in part mechanical, due to 

 underlying pressure or contact with an underlying body. It is probably 

 also chemical, whether the chemical interaction involves the molecular 

 interactions of apposed surfaces or the diffusion of substances of "at- 

 traction" over a larger distance of separation, because the extent of re- 

 action of the epidermis varies according to the exciting substance. 

 Foreign substances, whether "inert" or tissue taken from another ani- 



