THEORY OF THE DEVELOPMENT OF ORGANISMS 117 



and could show further, that he is able to determine 

 the mode of growth by cutting, bending, tying in a 

 horizontal position, and so forth/ For such reasons, 

 Yochting describes plants as masses of tissue, 

 practically plastic, and which may be moulded at 

 the discretion of the investigator. ' For instance, 

 in the case of Prunus spinosa, a branch may be 

 produced in place of a thorn by cutting a growing 

 shoot at the proper height, in spring. The buds 

 below the point where the cut was made turn to 

 shoots like the rest of the plant and complete the 

 interrupted growth, while on an uncut stem they 

 would have grown to thorns. Thus, the rudiment 

 of a thorn has been changed to that of a shoot ' 

 (Yochting). 



Although it is more difficult to carry out experi- 

 ments upon animals, some good instances are 

 known. If a piece cut from the stem of Antennu- 

 laria (a hydroid polyp) be placed vertically, in a 

 short time new branches and new ' roots ' spring 

 from it. In this case, again, the position of the 

 new growths is determined by the relation in which 

 the stem is placed to gravity. * The tentacles arise 

 only at the end turned towards the zenith ; the 

 " roots " from the parts directed towards the ground ' 

 (Loeb). 



A similar example may be taken from among 

 vertebrates. The notochord arises from a set of 

 cells which are in close relation with the fused tips 

 of the blastopore. By exposing developing frog's 

 eggs to abnormal conditions, I was able, in some 

 cases, to produce a hypertrophy of; one of the lips 



