SOME PROBLEMS OF REGENERATION. 20I 



grafted on to larger ones they may then regenerate, as Joest 

 has shown for the earthworm. 



There is a curious fact in regard to the development of small 

 pieces, namely, that they do not give rise, in some cases at 

 least, to the complete number of organs characteristic of the 

 form. For instance, Miss Peebles has found that a small piece 

 of Hydra forms only one or two tentacles. Why does this 

 reduction in number of organs take place } 



VI. 



We come now to some of the most important problems of 

 regeneration • — the external and internal conditions that deter- 

 mine whether or not a part will regenerate. The little that 

 we know in regard to the effect produced by external factors 

 is almost entirely due to Loeb's important experiments. A 

 hydroid Eiidendriiim failed to develop new heads when kept in 

 the dark, but when brought into the light the new heads quickly 

 appeared. Here it seems that light, in some unknown way, acts 

 as a stimulus necessary for the growth of the new head. In 

 another hydroid — Ttibidaria — Loeb has shown that instead 

 of a head a root will develop at the distal end of a piece if that 

 end be brought into contact with some fixed object, and con- 

 versely a new head will appear at either the proximal or distal 

 end if the end be freely surrounded by water. Here also we must 

 look upon the external agent as a stimulus that determines the 

 differentiation of the part. At present we can form scarcely 

 any idea of what this relation is between the external agent and 

 the organism. The vital question is, whether we can extend 

 our experiment so as to determine what this relation is, or have 

 we here a fact beyond which we cannot hope to go } 



Loeb has also described some further experiments on an- 

 other hydroid, Antenruilaria. A piece cut from the stem and 

 suspended vertically in the water will develop a new stem at 

 the upper end and roots at the lower end, regardless of whether 

 the upper end corresponds to the distal or proximal end of the 

 original form. Here gravity alone determines that the upper 

 end shall grow into a new stem and the lower end into a new 



