BIOLOGY 



nation. Such experiments, taken in connection 

 with similar ones performed upon developing em- 

 bryos, show conclusively that the living organism 

 has a very remarkable power of self-regulation, 

 and that the whole is something much more 

 than the mere sum of its parts. The laws which 

 govern these regulatory processes are at present 

 very little understood, and the results obtained 

 experimentally often appear at first sight to be 

 curiously capricious. Thus, if you remove the 

 eyestalk and eye of a crayfish by cutting it off 

 at one level, another eye and eyestalk will grow in 

 its place, but if you cut it through at another level 

 a feeler will develop instead. This is taken by 

 some to indicate that the eyestalk is really a modi- 

 fied biramose appendage which has undergone 

 change of function in the course of evolution, but 

 this does not help us to understand the mechanism 

 of the mysterious substitution. 



Closely akin to these experiments are those on 

 transplantation and grafting of organs. One of 

 the most remarkable of these again concerns the 

 vertebrate eye. It has been shown in certain 

 cases that if the outgrowth of the embryonic brain 

 known as the optic vesicle, and destined to give 

 rise to the retina, be removed from its normal 

 position and inserted beneath the skin on some 

 other part of the head, its presence there will cause 

 the development of a lens in the new situation, 



