''Polarity." 497 



tion is a factor in the rate of development the posterior piece 

 might develop a hydranth at its anterior end before the anterior 

 piece makes a hydranth at its posterior end, despite the slight 

 advantage of the material out of which the aboral hydranth 

 develops. This is, in fact, what occurs, although the difference 

 in rate is not very great and might be overlooked were not a 

 close watch kept on the pieces. Halves of the same piece must 

 of course always be compared rather than different pieces. 

 An experiment of this kind gave the following results: 

 Long pieces were tied at the oral and basal ends and were then 

 cut in two in the middle. After twenty-four hours five primordia 

 were present at the oral cut ends of the posterior pieces, and none 

 at the basal cut ends of the anterior pieces. Six hours later the 

 former had primordia in five pieces, the latter in three (but less 

 advanced). After forty-eight hours one of the posterior pieces 

 had produced an oral hydranth, and eight hours later four 

 hydranths were out on these pieces but no aboral hydranths as 

 yet on the anterior pieces. These results show that the hydranth 

 at the oral end of a piece (closed at the basal end) develops a little 

 sooner than does the hydranth at the basal end of a piece (closed 

 at its oral end). This difference can be safely attributed, I think, 

 to the difference in the direction of the gradation of the material 

 near the two cut ends. The results confirm an experiment of 

 King.i 



The Probable Influence of the Materials in the Circulating 

 Fluids on the Rate of Development. 



It has been shown by Driesch, Morgan, and Loeb that by 

 closing the oral end of a piece by a ligature the development of 

 the aboral hydranth is greatly hastened; so much so, in fact, that 

 it develops nearly as soon as an oral hydranth at the same level, 

 as described in the last section. What factors cause this accelera- 

 tion? It is clear that the suppression of the oral hydranth. is in 

 some way connected with the result, and it seems not unreason- 

 able to suppose that when the oral hydranth develops it draws 

 on the food supply and thus holds in check the aboral hydranth. 

 The problem is, however, complicated in several ways. In the 



*Biol. Bull., vi, 1904. 



