INTERNAL FACTORS OF REGENERATION IN ANIMALS 



61 



Since the red-pigment hypothesis does not explain the phenomenon 

 of the formation of the partial structures in tubularia, we must look 

 for another explanation. As the matter stands at present we can only 

 assume that there is a predisposition of a very small piece to form a 

 larger partial structure than a smaller whole one. This problem of the 

 method of development of small pieces of the stem of tubularia is fur- 

 ther complicated by the development in many cases of double hy- 

 dranths, or double parts of hydranths, as shown in Fig. 29, A-E. 



E 



FlG. 29. Tubularia mesembryanthemum. A. Short piece with hydranth at each end. B. Double 

 piece with one circle of proximal tentacles. C. Double piece with only two proximal tentacles. 

 D. Double proboscis with two sets of reproductive organs. - 3 . Double proboscis. 



The first form (Fig. 29, A) shows two hydranths turned in opposite 

 directions, that are united at their bases. Another form has only a 

 single circle of proximal tentacles between the two proboscides (Fig. 

 29, B-C\ In other forms there are only two proboscides, each with its 

 reproductive organs (Fig. 29, D\ and often there are simply two pro- 

 boscides united at the base (Fig. 29, E-F. Z \ It is the rule, even 

 in longer pieces, that a hydranth appears at each end of the piece, if 

 the piece is suspended or even lies on the bottom of the water ; but 



