THE FUNCTIONAL PERIOD OF DEVELOPMENT 



429 



An interesting half-way stage between chemical eflFects due to 

 contact, as in the determination of a lens by the eye-cup, and those 

 due to circulatory hormones, is seen in the perforation of the right- 

 hand side of the operculum in Anuran tadpoles during meta- 

 morphosis. As is well known, the rudiments of the fore-limbs 

 develop beneath the operculum, and while the left fore-limb makes 

 its way out through the open spiracle, the right protrudes through 

 a special perforation. After extirpation of the right fore-limb rudi- 



— p 



J 



Fig. 209 

 Perforation of opercular skin of Rafia paiustris, grafted on to the back, over pieces 

 of atrophying tail-muscle. The histolysis of the opercular skin leading to per- 

 foration is the same as that normally due to the atrophying gills, though slower. 

 a. Larva, showing graft of opercular skin {OG), perforated (P). h, Enlarged 

 view of graft showing atrophying tail-muscles seen through the perforation. 

 (From Helff, JoMr«. Exp. Zool. xlv, 1926.) 



ment, perforation of the operculum still occurs,^ thus demon- 

 strating that it is not due to mechanical pressure. Actually, it is a 

 substance liberated by the gills during their atrophy that is re- 

 sponsible for the perforation, as is shown by experiments in which 

 metamorphosing gills are grafted beneath the skin of the back and 

 cause perforation here too.^ Other atrophying organs, such as the 



1 Braus, 1906. ^ Helff, 1926. 



