THE UNIQUENESS OF THE INDIVIDUAL 



Contrast the state of affairs that has just been described 

 with another difference between the races that together con- 

 stitute the epidermal (or ectodermal) epithelia: the difference 

 between '"ordinary'' body skin and the compact, non-flaking, 

 perfectly transparent epithelium of the cornea. Here again, 

 the difference is of developmental origin; nor is it kept in being 

 by the fact that the corneal epithelium lives in an environment 

 very different from that of ordinary skin. The cornea is non- 

 vascular, moist and cool; ordinary body skin is vascular, dry 

 and (being dry) warmer than the cornea. Yet if corneal epi- 

 thelium is transplanted to an area formerly occupied by 

 ordinary body skin, and vice versa, the distinctive differences 

 between the two remain (Billingham and Medawar, 1950). The 

 property that distinguishes this case from the one just con- 

 sidered is this: that if the difference between corneal and ordin- 

 ary body-skin epithelium were not of developmental origin, 

 it could 7iot be reproduced within an individuaPs own lifetime 

 by difference of environment or of mode of use. The difference 

 will be established by developmental mechanisms, i.e. by the 

 appropriate segregations within the lineage of cells arising by 

 division of the zygote, or not at all. 



Let us call the difference between corneal and body-skin 

 epithelium a difference of Class A, and that between sole-of- 

 foot and body-skin epithelium a difference of Class B. To these 

 should be added a third category of difference, of Class C 

 (Abercrombie, 1952): one which is not developmentally pre- 

 fabricated, but which may arise purely from difference of 

 environment or of use.* The pigmentary cells (melanocytes) of 

 the epidermis of the two sides of the face or the two arms may 



* [This rather arid terminology was based upon that of an article 

 published in New Biology, 11, p. 10, 1951. Much better, because self- 

 explanatory, is C. H. Waddington's {loc. cit., 1953): Class C adaptations 

 are 'exogenous'. Class A 'endogenous', and Class B 'pseudo-exogenous'. 

 I was not able to benefit from these suggestions, because the present article 

 was two years 'in the press'.] 



86 



