REGENKRA'IIOX 97 



epidermis, and finally the development of smooth muscle fibres 

 takes place.' * New integumentary sense-organs arise in a sim- 

 ilar way. A number of young cells become arranged so as to 

 form a rounded mass in the deeper portion of the newly-formed 

 epidermis : these then become elongated in a direction vertical to 

 the surface of the epidermis, the central element undergoing 

 differentiation into sensory cells, while the peripheral ones form 

 an investment around these. 



It is evident that the process is rendered more complicated in 

 this case by the fact that the young epidermic cells, formed by 

 the proliferation of those already present, give rise to cells of 

 various kinds, viz., to ordinary epidermic cells, to gland cells, 

 and to sensory and * investing ' cells ; and the complication is 

 further increased by all these cells being arranged and localised 

 in a perfectly definite and more or less prescribed manner. We 

 certainly must not assume that the formative cells which undergo 

 these various differentiations are really identical, although they 

 may appear so. It cannot possibly depend on external influ- 

 ences alone whether one of these subsequently becomes trans- 

 formed into a horny, glandular, or sensory cell ; for we cannot 

 assume the existence of such a regular and localised difference 

 in the external influences. The various differentiations of the 

 formative cells must therefore depend on their own nature — that 

 is to say, on the determinants contained within them, which have 

 hitherto been latent but which have now become ripe, and have 

 impressed a specific character upon each cell. Tliese formative 

 cells f/mst have contained differoit sorts of determinants from 

 the first. 



Fraisse compares the processes which can be observed in the 

 regeneration of the skin in Amphibia with those which occur in 

 the embryogeny of this class, and shows that they are essentially 

 similar. We shall therefore be justified in imagining these proc- 

 esses — which are invisible to us even under the highest powers 

 of the microscope — to be homologous with those which take 

 place during the development of the embryo. 



We can thus further assume that the stratified cells in the 

 'mucous layer" of the epidermis, although apparently all alike, 



* Cf. Fraisse, ' Die Regeneration von Geweben u. Organen bci den Wir- 

 belthieren, besonders bei Amphibien u. Rcptilien." Cassel and Berlin, 

 1885. 



