452 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



only on the tissues of the same individual, but also on tissues transplanted 

 from another individual of the same, or even of a different species ; if it is 

 present at the time just preceding transplantation in a larger quantity in the 

 host than in the donor, the metamorphosis of the transplant tends to be ac- 

 celerated ; but if present in a larger quantity in the donor than in the host, 

 then a relative retardation in the metamorphosis of the transplant, as compared 

 with the metamorphosis which would have taken place in the donor, is apt to 

 occur. 



However, in addition to these factors, others which are present in the 

 transplant influence the character and time of metamorphosis. Among these 

 latter, primary factors inherent in the structure of the tissues, and secondary 

 ones depending on variable environmental conditions, can be distinguished. 

 Thus, the iris of the eye in salamander and also the gills in urodeles undergo 

 certain changes apparently under the influence of specific substances, which 

 become potent some time previous to the onset of metamorphosis, but the 

 mode of action is influenced by specific characteristics of the tissues. Even 

 skin from different surface areas of the same animal may differ as to its 

 reactivity to these substances. According to Lindemann, the skin of the tail 

 of frog larvae will undergo absorption during metamorphosis, and this takes 

 place irrespective of whether the skin has been left in its normal place or 

 whether it has been transplanted into other parts of the body surface. On the 

 other hand, dorsal skin will remain unchanged, even if transplanted into a 

 place which undergoes retrogressive changes during metamorphosis. The con- 

 dition of the tissues of the donor may modify the metamorphosis in still an- 

 other way : if the donor organism at the time of transplantation has reached 

 a stage nearer to metamorphosis than the host, the transplant has a tendency 

 to metamorphose at an earlier date than the host; if, on the contrary, the 

 donor is still farther removed from the stage of metamorphosis, the trans- 

 plant tends to require a longer time before metamorphosis can take place. It 

 seems therefore that preceding the processes occurring during metamorphosis 

 there are preliminary changes in the tissues, which make the latter more re- 

 sponsive and gradually sensitize it to the substances causing metamorphosis, 

 and this process of sensitization requires a certain time. It is possible that the 

 sensitizing substance is identical with the metamorphosis-inducing substance. 

 We may then assume that this substance gradually accumulates in the organ- 

 ism, combines with the responsive tissues and thereby makes them ready for 

 metamorphosis, which takes place after a certain point of tissue saturation has 

 been reached and after the hormone has had a chance to act on the tissues for 

 a sufficient length of time. The possibility also exists that the sensitizing sub- 

 stance differs from the metamorphosing substance and merely makes the tis- 

 sues receptive to the action of the latter substance. However that may be, 

 a tissue thus sufficiently prepared undergoes metamorphosis after transplanta- 

 tion, even without the presence of the active metamorphosing substance in 

 the host, whereas a tissue not fully prepared or sensitized is not sufficiently 

 responsive even if the metamorphosing substance of the host is fully active. 

 Such a transplant will, therefore, not metamorphose synchronously with the 



