2/2 T. H. MORGAN. 



ten hours cut in two in the middle. Comparing the rates of de- 

 velopment of the oral ends of the oral halves with that of the oral 

 ends of the basal halves it appears that the latter are slower, 

 but there is evidence that the retardation may be somewhat less 

 than the ten hours difference in initial start. The basal halves in 

 this experiment are also somewhat behind the basal halves in the 

 last experiment, which seems to show that the general changes 

 in the excised pieces that go towards oral development decrease 

 from the cut ends inwards. 



It has not seemed necessary to give the details on which these 

 general conclusions are based. The nature of the case makes it 

 difficult to obtain results as definite as one might wish, despite 

 the precautions that were undertaken to make the conditions as 

 uniform as possible. The general conclusion that changes take 

 place in the piece as a whole, after its removal, that are in the 

 direction of hydranth formation, seems fairly certain. Less cer- 

 tain perhaps is the evidence to show that when the oral end is 

 tied similar changes take place in the pieces that accelerate basal 

 development in regions beyond the hydranth forming region, but 

 this conclusion too is, I think, quite probable. The nature of 

 these changes is not revealed. 



THE DYNAMIC FACTOR IN EGG-DEVELOPMENT. 



Students of the processes of regeneration have without excep- 

 tion made use of the term polarity to express a directive factor 

 observable in their results, and to this factor is sometimes as- 

 cribed an active role as a controlling influence, at other times the 

 term is used descriptively merely as a statement that the new 

 structures are directed in the same way as the part removed. In 

 both respects the word has been useful, however vague our con- 

 ception of what polarity may be. Our analysis of the process 

 has now gone sufficiently far, I think, to justify us in an attempt 

 to come to closer quarters with the term. Without reviewing 

 the opinions that have been expressed as to the nature of polarity, 

 I shall try to contrast two views of its nature that seem to me to 

 represent the two main lines that speculative thought has followed. 

 It should be noted that the term is used equally by students of 

 embryology and by students of regeneration. The former 



