



392 1 . H. Morgan. 



and of heteromorphosis that may be very briefly touched upon at 

 this time. Although I have not hesitated in earlier papers to 

 speak of polarity as a factor in regeneration, I have always 

 tried to be careful to state that we are really entirely ignorant in 

 regard to its nature. When we see the polarity suddenly reversed 

 in cases of axial heteromorphosis it appears that this ought to 

 throw some light upon the nature of the factor itself, yet despite 

 the numerous surmises that have been made of a material, — chem- 

 ical, or electrical nature — we still remain totally in the dark as 

 to what factors determine the stereometrical relations of the new 

 part. The following facts appear, nevertheless, to have an im- 

 portant bearing on this topic, and while they do not offset an imme- 

 diate solution of the problem, yet they may point in the direction 

 in which an analysis may ultimately be undertaken. 



In the more highly specialized forms the question of what re- 

 generates appears, in part, to be connected with the nature of the 

 material, or with the kinds of the material that give rise to the 

 new cells, and the relation of direction is less apparent. The tail 

 of a tadpole regenerates only a tail, even at its anterior end. The 

 same appears to be true for the leg of the salamander from certain 

 results that I have obtained, which are as yet unpublished. In 

 the earthworm as shown by Morgan^ and by Dimon^ the 

 regeneration of an orthomorphic head is connected with the 

 presence of the anterior structures of the worm, while from the 

 part containing the intestine — including by far the greater length 

 of the worm — only a tail is, as a rule, regenerated, even from the 

 anterior cut end. In these cases it appears that the nature of the 

 material must decide the character of the new part, and the polar 

 relations do not come conspicuously to the front, although that 

 something of the sort still enters into the problem is shown by the 

 slower rate, and, in some cases, by the less perfect form of the 

 heteromorphic growth. 



On the other hand, in less specialized forms the polar relations 

 appear to play a more conspicuous role. In Lumbriculus a head 



'Anatomischer Anzeiger. Bd. 15, 1899. 



^Journal of Experimental Zoology. Vol. I, No. 2, 1904. 



