HETEROMORPHOSIS 129 



which Allman and Dalyell experimented, behaves typically 

 differently from Tubularia mesembryanthemum, upon which 

 I made my experiments. A certain difference seems, in- 

 deed, to exist. Dalyell states that the stem of the Tubularia 

 indivisa bends upward, when laid horizontally ; this I have 

 not observed in Tubularia mesembryanthemum. 



Yet I do not believe that the conditions determining the 

 form of Tubularia indivisa differ in principle from those in 

 Tubularia mesembryanthemum. For Dalyell notes that he 

 once observed the growth of polyps from both ends of a 

 piece cut from the middle of a Tubularia indivisa. "It 

 may be conjectured that the summit of both had originally 

 constituted a single embryo, which by partition deve]oped 

 into two, becoming progressively symmetrical in maturity." 1 



To explain the formation of a head at both ends of the 

 stem in the single case just described, Dalyell therefore 

 assumes that the new head divided in the course of its 

 development. Such a division would, in consequence, always 

 have to occur in the case of Tubularia mesembryanthemum, 

 which without exception forms a head at both ends, if both 

 ends are surrounded by water and have a sufficiently great 

 diameter, and a dividing embryo would therefore have to 

 exist in every piece of the stem of Tubularia mesem- 

 bryanthemum. Even if one were willing to consider this 

 hypothesis, it yet could not be made to harmonize with 

 Allman's theory of polarity; for, according to this theory, 

 both embryos would necessarily have to develop always 

 at the same end, namely, at the oral one ; yet I have never 

 found two heads to develop here side by side. 



2. I might mention that it is possible apparently to ob- 

 tain such results in Tubularia mesembryanthemum as All- 

 man describes, if the stems used in the experiments are cut 

 off close to the root, and if care is taken, in choosing the 



1 Loc. cit., p. 35. 



