DEVELOPMENT OF EPITHELIUM. lOT 



superficial cutaneous inflammation (eczema and vesication) tliey 

 detected migrating cells in all the younger strata of epithelium, 

 cells precisely similar to other amceboid corpuscles visible in the 

 papillary body (fig. 35). One fact, and one only, is being 

 continually urged against this hypothesis — of the fact itself 

 there can be no doubt — it is this : new epithelium, as e.g. after 

 partial denudation of any epithelial surface, originates by prefer- 

 ence, perhaps even exclusively in connexion and immediate con- 

 tiguity with pre-existing epithelium. It would seem, therefore^ 

 since fission of the older elements cannot be shown to occur, that 

 we are driven to assume that an embryonic cell can only develope 

 into an epithelial cell when it is in contact with one of the latter 

 kind. We must perforce adopt the theory of a sort of ^'epithelial 

 infection.^'' This theory indeed would necessitate a twofold appli- 

 cation ; for supposing such infection to occur when embryonal 

 cells, colourless blood-corpuscles, &c., are brought in contact 

 with a permanent epithelium, it must likewise occur when, con- 

 versely, epithehal elements are brought in contact with a tissue 

 composed of embryonic cells. The latter process indeed may 

 actually be observed in the metastasis of cancer to lymphatic 

 glands. 



§ 84. This brings us to the end of our general summarv of 

 normal growth. We have seen, that apart from the first founda- 

 tion of the various organs (from which we learnt that any and 

 every tissue might originate from embryonic tissue), their actual 

 growth depends but in small measure on fissiparous multiplica- 

 tion of their specific tissue-elements. We found this mode of 

 increase in the gland-cells, in the fibres of striped muscle ; 

 to some extent also in cartilage ; and we had grounds for 

 suspecting that it also occurred in nerve-fibres. On epithe- 

 lial surfaces there seemed to be need of an ''- epithelial infection," 

 or at least of some sort of action of the older cells upon the 

 immigrants. Everywhere else, the intermediate apparatus of 

 nutrition, with its faculty of generating embryonic cells wherever 

 required, sufficed of itself to maintain normal growth. It may 

 be that it fulfils this function as a corollary of those obligations 

 which are incumbent upon it in its nutrient capacity, allowing 

 a certain number of colourless corpuscles to accompany the usual 

 supply of fluid pabulum into growing organs, these corpuscles 

 serving forthwith as materials for the constructive process. 



