The Organization of the Egg 195 



end of another ; two Hydras may be united by either anterior 

 or posterior ends, or one Hydra may be grafted onto the 

 side of another. The results differ depending upon the condi- 

 tions of the experiment, and the species of Hydra employed; 

 but the general result is that a process of regulation ensues 

 whereby a new animal is formed, similar in size and pro- 

 portion to the normal individual. One of the most interest- 

 ing results bearing on the question of predetermination of 

 parts is that obtained by grafting two Hydras by their 

 anterior ends and then cutting off the posterior end of one 

 near the graft line. In this case a new head forms on the 



FOUR-LEGGED TADPOLES 



Produced by transplanting the limbs from one tadpole to another. 

 After Harrison, " Journal of Experimental Zoology," Vol. 4. 



(originally) posterior end of the graft, where a head, in the 

 ordinary course of events would never develop. 



Some of the most interesting grafting work of recent years 

 has been done by Harrison, in connection with studies on the 

 developing nerve fiber. Two positions have been held on this 

 question one, that the axone of the nerve cell, the conducting 

 part of the nerve fiber, arose in situ from surrounding cells ; 

 the other, that the axone was an outgrowth from the nerve 

 cell itself. The latter view appears to have been definitely 

 established by Harrison. Our interest here however centers 

 primarily upon certain secondary results of Harrison's work 

 rather than upon the question of nerve fiber development. 

 In these experiments Harrison has shown that limb buds can 

 be transplanted from one tadpole to another, the tail of one 



