§ 1] UPON THE KATE OF GROWTH 317 



rence permits us to believe that it has an importance, if not for 

 growth, at least for development ; there is, however, no direct 

 evidence that it is generally necessary to organisms. Since it 

 is closely allied to chlorine, the question whether it may replace 

 chlorine in growth has been tested. Nencki and Schoumow- 

 SiMANOAVSKT ('94) have found that in the higher animals 

 the bromides can replace the chlorides to a limited extent, but 

 are clearly less advantageous. Altogether the importance of 

 bromine for growth is slight. 



Iodine. — This element is of wide-spread occurrence in organ- 

 isms, probably as a constituent of organic molecules, for it is 

 found in plants, especially in some species of Fucus and Lami- 

 naria (cf. Eschle, '97); in invertebrates, especially in sponges 

 and the stem of Gorgonia ; and in vertebrates, especially in 

 mammals, where it has recently been shown to be most im- 

 portant for growth. It has long been known that mammals 

 which have been deprived of the thyroid gland acquire a weak 

 condition of body known as myxoedema. This effect has been 

 accounted for by the loss, to the organism, of a substance 

 elaborated in the thyroid gland ; for when the thyroid gland, 

 or a docpction of it, is fed to the animal it recovers to a certain 

 extent its normal condition. The nature of this substance has 

 been investigated by Batjmann and Iloos ('96), who find that 

 it is a compound of iodine — thyroiodine ; for when thyro- 

 iodine is fed to the myxoedema patient, the same favorable 

 result ensues as follows feeding upon the gland. 



Fluorine is found rather widely distributed among verte- 

 brates in very minute quantities. It forms about 1.3% of the 

 total ash of bone. It is present also in the hen's egg, being 

 more abundant in the yolk than in the albumen (Tammann, 

 '88). Since it is chiefly found in the body as a constituent of 

 bone, possibly in the form of the mineral apatite, Caj^Fg (P04)g, 

 it may very well be that its chief importance is in the consti- 

 tution of this formed substance. According to Beandl and 

 Tappeiner ('92) the normal amount in the body may be 

 greatly increased by feeding small quantities of sodium fluoride 

 during a long period. Altogether, we have no reason for 

 thinking that fluorine is essential to the growth of organisms 

 in general. 



