6O REGENERATION 



fluid had been added to the wall where the hydranth is produced. 

 The globules disappear in the region of the new hydranth, but, I 

 think, it can be shown that they do not form any essential part of the 

 hydranth. They may be found stuck together in a ball that lies in the 

 digestive tract of the new hydranth, and when the hydranth is fully 

 formed the pigment is ejected, as Stevens has shown, through the 

 mouth. 



The development of the new hydranth begins several hours be- 

 fore the red-pigment globules have disappeared from the circulation. 

 The walls in the region of the future hydranth begin to thicken, 

 and, later, pigment develops in the endoderm of this region. The 

 new pigment is formed in the new cells of the endoderm, and does 

 not come from the circulating globules, as shown by the development 

 of very short pieces of the stem. In these the amount of new pig- 

 ment that develops in the new hydranth may be far greater than that 

 in the whole original piece (Fig. 30, D\ and in this case there can be 

 no question but that new pigment is made in the endodermal cells of 

 the hydranth. The formation of a hydranth, that usually takes place 

 after another twenty-four hours, from the basal end of a long piece, 

 shows that a hydranth may develop when there are no granules in 

 the circulating fluid. These basal hydranths may contain as much 

 pigment as do the distal ones. 



Driesch suggested that the red-pigment in the circulating fluid 

 determines quantitatively by its presence how much of a hydranth 

 is formed, or the size of the hydranth in relation to the rest of 

 the piece. There seems to be no evidence in favor of this view 

 and much against it. Loeb has not stated specifically whether 

 he means that it is the pigment in the circulating fluid or that 

 in the walls which acts as a formative stuff; the presumption is 

 that he meant the latter. An examination of the piece during regen- 

 eration gives no evidence in favor of the view that the pigment moves 

 into the region of the new hydranth. On the contrary, it remains 

 constant in amount at all points except where the new hydranth is 

 developing, and there is in this region unquestionably a large develop- 

 ment of new pigment. 



The evidence for and against the idea that the red-pigment of 

 tubularia is a formative stuff, or even building material, has been 

 considered at some length, because it is the only case in which the 

 hypothetical formative stuff has been definitely located in a specific, 

 recognizable substance that can be followed during the process of 

 regeneration. It is well, I think, to give the question full considera- 

 tion, especially as the hypothesis often appears to give an easy solu- 

 tion of some of the problems of regeneration. In a later chapter the 

 subject will be more fully treated. 



