The Phenomena of Polarity in Tiihularia. 589 



substances toward the place where they are consumed, such a 

 movement is the effect of the development and not its cause. 



In the case of Tiihularia it is clear from many experiments that 

 the rotary circulation in the stem (which is the result of ciliary 

 action) can not be made to account for the polarity of the stem. 

 The red pigment does not appear to be a nutritive or a formative 

 material. It seems to bear no other relation to the regeneration 

 than that of a waste product. The remaining material of the ridges 

 that is set free along with the pigment may be, and probably is, a 

 nutritive substance that is made use of by the developing hydranth. 



Some other possibilities involved in my hypothesis may be 

 briefly mentioned. Experiments have shown that when a polyp 

 develops on the aboral end some change takes place, especially 

 in the region immediately behind the polyp, of such a sort that 

 we may say the "polarity" of that region has been changed. 

 This means, on my view, that by the development of a polyp on 

 this end the region behind the polyp has been also changed and 

 is more like the region behind a normal polyp. Consequently 

 a short piece in this region may produce an aboral polyp (in the 

 original sense) before an oral one. How far this influence extends 

 is not conclusively shown, but the effect is certainly stronger 

 just behind the new aboral polyp, and it probably diminishes 

 rapidly in the original oral direction. 



A possible objection to my view may be made on the following 

 grounds: If pieces of medium length contain enough food 

 material to produce oral polyps then, if food stuff were made 

 throughout the piece, there should be an excessive amount in 

 long pieces, and aboral as well as oral polyps should develop in 

 the latter, which is not the case. The answer to this objection 

 is that the change in the food stuffs that makes them soluble 

 probably takes place only, or largely, in the region where the 

 polyp is developing and not throughout the piece. The soluble 

 material set free at the aboral end as well as at the oral end is 

 utilized at the oral end, because that end develops first. The 

 actual observations suggest, if they do not prove, that the available 

 material may be added to by the protoplasm and its food contents 

 set free from the endodermal ridges. 



