PHYSIOLOGY OF REGENERATION IN TUBULARIA. I 57 



This observation sets aside, I think, any attempt to explain polyp- 

 formation on the physiological basis of the direction of the cur- 

 rents. The inadequacy of such an explanation is also readily 

 seen when short pieces with "double" partial hydranths are ex- 

 amined, in which the continuous current is readily demonstrated. 



" Polarity." 

 In an earlier paper I have pointed out that experiments show 

 that there exists a graduation in the materials of the stem 

 of Tubularia, and that on this as a basis we may attempt to 

 adjust our conception of polarity. This gradation can be seen 

 structurally in the change in the thickness of the walls and in 

 the histological character of the cells of different levels. Physi- 

 ologically it is shown in the more rapid regeneration of the cut 

 surfaces the nearer they are to the distal end. This difference in 

 rate I supposed might be due to the greater amount of " hydranth- 

 forming " materials 1 near the distal end, or more correctly to the 

 less differentiation of the more distal parts. It was perhaps 

 unfortunate to have used the words hydranth-forming mate- 

 rials, for it might readily be inferred that I meant to refer to 

 formative materials or substances as such, i. e., independently of 

 the differentiation that decreases in amount distally. A care- 

 ful perusal of the text, especially of later papers, will show that I 

 had not so much in view the presence or absence of peculiar and 

 unknown " stuffs " (a view I have often disputed) as the direction 

 that differentiation had taken in different regions. The more 

 distal parts of the stem are less specialized as storage and sup- 

 porting tissues than the basal parts. The distal region, having 

 as it were, less to undo, develops more quickly into a polyp. 

 This sort of difference of materials, with its concomitant physi- 

 ological differences, may furnish a basis for that particular condition 



1 By hydranth-forming materials I meant not reserve stuffs or " organ-forming sub- 

 stances ' ' in the sense in which some embryologists of the preformation school use 

 these terms, but rather materials that have already been differentiated into a part 

 of the anterior end of the body or head. The phrase is more applicable to 

 cases where the head is not sharply marked off from the trunk as in the earthworm, 

 in planarians, etc. That the direction in which differentiation of a tissue has taken 

 place is an important factor in determining the future course of regeneration of that 

 tissue is familiar. By generalizing this fact I have attempted to make it the basis 

 for the phenomena of polarity. 



