Vertebrates 



689 



Here it would seem possible to differentiate 

 sharply, but the line is not so clear as one 

 would think, as the subject when analyzed 

 yields a difficult series of interrelations, some 

 of which occur during all the normal states 

 of use and repair, shading to an acute res- 

 toration of parts which are badly damaged 

 if not completely ruined by a physiological 

 overload. The step between a normal and 

 a pathological process is so minute that it 

 is hard to determine except by arbitrary 

 rule which one belongs in which category. 

 The same condition exists here, and in the 

 following evaluation of the repetitive proc- 

 esses it is by no means simple to place them 

 under a hard and fast series of definitions. 



Epidermal Replacement. The stratimi germ- 

 inativum, or malpighian layer, is constantly 

 giving rise to new elements which are re- 

 placing the superficial parts. In reptiles the 

 replacement may be gradual as in the Che- 

 Ionia and Crocodilia, or in rapid bursts of 

 cornified epithelium as in some lizards and 

 serpents, in which the surface cornification 

 may be a complete cast rigidly mirroring 

 every detail of the underlying parts. Even 

 the corneal layer of the eye is sloughed at 

 an ecdysis. Here again the process has a 

 superficial resemblance to that occurring in 

 arthropods. The glands in association with 

 the cuticle, and the individual scales of 

 which the cuticle are composed, all have 

 their basic levels of regeneration through 

 the activity of the stratum germinativum. 



The skin of mammals has a similar con- 

 stitution and replacement power but the 

 surface epithelium is sloughed slowly, and 

 the replacement is not noticeable unless there 

 is an actual denudation of a large area with 

 subsequent restoration. Bishop ('45), attack- 

 ing this problem experimentally, finds by 

 biopsies of varying thickness that the limi- 

 tation of healing and scar formation can be 

 fairly definitely marked out. Complete re- 

 generation results when removal does not 

 go below the reticular layer. Fibrosis is in- 

 hibited if the papillary layer is left intact. 

 In the birds the situation is quite different, 

 for they seem to have the attributes of both 

 groups. Feather regeneration occurs after 

 feather removal, and the process of reforma- 

 tion is essentially similar to the embryonic 

 process. Samuel (1870) gave the first nearly 

 complete picture of the process. Lillie and 

 Jvihn ('32) showed the complete picture of 

 feather regeneration and the pattern of the 

 regenerating feathers. 



In addition to the usual process, birds 

 have a periodic moulting in many cases and 



this process is similar to the ecdytic phe- 

 nomena in reptiles. They have, however, 

 little replacement of skin as the keratiniza- 

 tion does not seem to reach the same degree 

 as it does in mammals. Moreover, birds 

 possess in the beak an vmusual integumen- 

 tary appendage, with the capacity to regen- 

 erate a large proportion of this structure 

 after injury or removal. Scales and spurs 

 also can be replaced. 



Hair growth and that of claws, hooves 

 and nails are relatively constant and do 

 not call for special consideration. The growth 

 of horns, however, proceeding as it does in a 

 very definite way in the forms which possess 

 the epidermal types, is really a remarkable 

 case of repetitive restitution. It was studied 

 in detail by William Harvey and John Hun- 

 ter. The correlation between the testis and 

 the antlers was made very clearly because 

 castrates in which the horns are shed do 

 not regain the increasing branched antler 

 but the rudimentary form. 



The glands in connection with the in- 

 tegument — sebaceous, sweat, ceruminous, 

 mucous and the necrobiotics — all have the 

 potentiality for comparatively rapid regen- 

 eration. The mammary gland was investi- 

 gated quite early, and it was found that the 

 nipple does not regenerate, nor will the 

 gland regenerate if the organ is completely 

 removed. If a part is left, however, the work 

 of Gardner and Chamberlin ('40-'41) shows 

 the amount of reorganization which can 

 occur. The tubular ducts and the glandular 

 epithelium both undergo amazing phases of 

 growth and reconstitute an active gland 

 from its involuted condition in a short time. 



While considering the general situation 

 in epidermal structures, it might be well to 

 digress for a moment to see what the super- 

 ficial reactions are during wound healing. 

 Epithelium in general gives rise to both the 

 glandular and the surface types. Occasion- 

 ally one type may form the other, but they 

 usually run true to rule. Peters (1885) found 

 that after the cornea is scraped the wound is 

 first covered by lymph which forms a clot 

 through which the epithelium becomes ac- 

 tive by (1) spreading movement and (2) 

 cell proliferation. Movement by spreading is 

 the initial reaction to the produced defect. 

 In the entire contiguous cell area the cells 

 are amoeboid and spread over the denuded 

 area. Mitosis resulting in cellular prolifera- 

 tion begins about 20 hours after the injury, 

 and here also the reaction extends to cells 

 quite removed from the area of injury. 

 The same process seems nearly universal in 



