478 E. J. LUND 



Rare individual exceptions to the above sequence of events 

 occur in various experiments. These facts are therefore iden- 

 tical with what is well known to occur in pieces cut from the 

 stem of Tubularia as shown originally by Driesch« and later by 

 Morgan,^ Child/ Hyman/ and others ^ 



In vigorous material under optimal conditions for regenera- 

 tion internodes of Obelia usually formed hydranths at both ends 

 in 100 per cent of the pieces. The control, plate lA, shows a 

 typical result. Whenever stolons did form, they appeared 

 many hours after the appearance of the hydranth of the same 

 piece, and then only, so far as was observed, at the basal end of 



^ Driesch, H., Studien liber das Regulationsvermogen der Organismen. 

 II. Quantitative Regulationen bei der Reparation der Tubularia. Arch. f. 

 Ent.Mech., 1899, Bd. 9, S. 103. 



^ Morgan, T. H., Hydranth formation and polarity in Tubularia. Jour. Exp. 

 Zool., 1906, vol. 3, p. 501. 



^ Hyman, L. H., The axial gradients in Hydrozoa. III. Experiments on 

 the gradient of Tubularia. Biol. Bull., 1920, vol. 38, p. 353. 



^ Differences in the time which it takes for basal and apical hydranths in such 

 regenerating pieces of hydroid stems (Tubularia) to reach complete differentia- 

 tion have been interpreted by some to mean that differences in rate of develop- 

 ment or 'rate of metabolism' occur at the two regenerating ends, and that this 

 difference in 'rate of metabolism' determines the difference in the time for com- 

 plete differentiation of the hydranths. This inference, it seems to me, is doubt- 

 ful in the absence of adequate evidence. For, after reading the papers by various 

 workers on this problem, I am unable to find in any of them any clear statements 

 or data which show that the difference in time between cutting of the pieces 

 and maturity of the hydranths is not due to a delay in the initiation of polyp 

 formation. In fact, one is led to a somewhat contrary conclusion when reading 

 Morgan,^ page 506, who states: " . . . . experiments .... seem 

 to show that as a rule no develpoment, or very little, takes place for a time at 

 the aboral end" (of pieces of Tubularia); again, page 509: "The stimulus once 

 received" (by the aboral end) " . . . . the development can go on si- 

 multaneously with that of the oral polyp, neither suffering retardation." It is 

 obvious that if differences of delay in initiating the differentiation occur, then 

 hydranth formation as such might even be qually, or more rapid at a basal than 

 at an apical end of a piece. This is a point which needs further critical quanti- 

 tative study before we can speak with assurance of differences in rates of dif- 

 ferentiation or metabolism of basal and apical ends. This consideration as to 

 the proper use of the term 'rate' does not, however, affect the interpretation' 

 of the results and conclusions in the present paper. We are only concerned here 

 with the possibility of electrical control of the mechanism in the regenerating 

 piece which determines morpholgical polarity. 



