Il6 HOW ANIMALS DEVELOP 



which pass through a larval stage. In the frog it has 

 been shown that it is a hormone which makes the 

 tadpole metamorphose into an adult frog. The 

 hormone in question is produced by the thyroid 

 gland, which Hes in the neck in man, and gives rise 

 to a form of goitre if it becomes too big. It is interest- 

 ing to find that human thyroid hormone can be 

 effective in causing metamorphosis in frogs, so the 

 hormones are not special for each species of animal. 

 It is probable that the moulting and metamorphosis 

 of insects is also controlled by hormones, but so far 

 no vertebrate hormone has been found which will 

 produce the effects. 



The chemical constitutions of some of the hormones 

 have already been worked out, and the extraordinary 

 fact has been discovered that it is sometimes possible 

 to substitute for a natural hormone various other 

 substances which are chemically rather unlike it. 

 Perhaps the most striking example is in one of the 

 numerous hormones concerned with regulating the 

 development of the sexual organs. A whole series of 

 substances are known which can be substituted for 

 this hormone, which is called oestrin. All these sub- 

 stances have certain points in common, although 

 they are quite dissimilar in other details. Moreover 

 some of them can do quite other things ; some can 

 act like the vitamin D, in preventing animals getting 

 rickets ; some of them can cause cancer to develop 

 if they are painted on to the skin constantly for 

 months; and some of them can actually act as 

 evocator-substances and cause the induction of 



