428 



THE PREFUNCTIONAL AS CONTRASTED WITH 



These regional differences in reactivity are established very early 

 (Schwind, J. Exp. Zool. Lxvi. 1933). The relation between hor- 



t.m 



'♦ 1- t 



1 BV 



< : 



L 



;-— E 

 — sc 





SBS 



EL 



P 





2 BV 





■;ih~E 

 "i.:- ss 



- ~~sc 



- --T_w E 

 '•_/^-^-SS 



K"^--sc 



^-'^^.^. 



3^ 



BV 



SBS 



EL 



Fig. 208 

 The perforation of the operculum in the frog {Rana claniitans). Sections 

 showing the histolysis leading to normal perforation, i, First sign of histolysis 

 (at X). 2, Histolysis well under way; the stratum compactum and stratum 

 spongiosum have become interrupted; there has been a marked invasion of 

 the area by lymphocytes, and the skin in this region is decreasing in thickness. 

 3, The skin is reduced to the epidermis, which the fore-limb then ruptures. 

 BV, blood-vessel; E, external epidermis; EL, epidermal lining of branchial 

 chamber; P, pigment; L, lymphocytes; SC, stratum spongiosum; SBS, con- 

 nective tissue; SS, stratum spongiosum; X, site of histolysis. (From Helff, 

 jfourn. Exp. Zool. xlv, 1926.) 



mones and growth-gradients is shown by studies on regeneration- 

 rate and hormone-susceptibility in birds' feathers. ^ 



Lillie and Juhn, 1932. 



