THE OUTER AND INNER TISSUES OF ANIMALS. 289 



enough to destroy the tegumentary structures, these are never 

 reproduced. The 'puckered surface formed where an ulcer 

 heals, consists of modified connective tissue, which, as the 

 healiug goes on, spreads inwards from the edges of the ulcer 

 — some of it, perhaps, growing from the portions of connective 

 tissue that dip down between the muscular bundles. This 

 connective tissue, mark, out of which is thus constituted the 

 make-shift skin, is normally covered by both the epidermis 

 and that stratum of indifferent tissue from which the growth 

 proceeds in opposite directions — is the inner layer that grows 

 inwardly. What has happened to it ? It has now become 

 the outermost layer. And how does it comport itself under 

 its new conditions ? It produces a layer that plays the part 

 of epidermis and grows outwardly. For since the surface, 

 subject to friction and exfoliation, has to be continually 

 renewed, there must be a continual reproduction of a super- 

 ficial layer from a layer beneath. That is to say, the con- 

 tact of this deep-seated tissue with outer agencies, produces 

 in it some approach towards that composition which we find 

 universally characterizes outer-tissue — a protomorphic layer, 

 which differentiates in opposite directions. But while we see 

 under this exposure to the conditions common to all integu- 

 ment, a tendency to assume the structure common to all 

 integument, we see no tendency to assume any of the 

 specialities of tegumentary structure : no rudiments of glands 

 or hair sacs make their appearance. 



This apportionment we shall see the more reason to accept 

 as approximately expressing the truth, on remembering that 

 the mode of differentiation of outer from inner tissues which 

 is common to all animals is common to all plants ; and 

 on observing, further, that the more special interpretation 

 suggested as not improbable in the case of plants, is not 

 improbable in the case of animals. For as it was argued 

 that in plants the forces evolved from within the organism, 

 and the forces falling on it from without, must have some 

 place between centre and surface at which they balance ; and 



