RE GENERA TION. 205 



cells have a different molecular structure from those derived 

 from other regions, we might, by combining the two assump- 

 tions, escape from the dilemma. On the first assumption, that 

 the cells of each new layer or new organ have received their 

 molecular character from the old part, and in each kind of 

 tissue are all more or less alike, we are driven to assume, on 

 the second assumption, that the form of the new part deter- 

 mines subsequent changes in the molecular composition, and 

 the material is so changed at each point that its arrangement 

 produces the foundation of a new head.^ 



Before we follow further this line of thought let us examine 

 those cases in which the entire piece is changed over into a new 

 organism. If a piece is cut from the middle of the body of 

 hydra, it closes in at both ends and a cylindrical form is the 

 result. If the piece owed originally its characteristic form {i.e., 

 as a piece of the body of hydra) to its molecular structure, it 

 would have this same character after separation ; and in giving 

 rise to a new hydra it does not develop new tissue at the two 

 ends like that lost, but, on the contrary, after a short time the 

 old piece itself assumes the form of a new hydra. If we hold 

 to Sachs's view, that wherever a new form arises there has been 

 an antecedent change in the material, we must conclude that 

 in the piece of hydra the material has rearranged itself into a 

 new whole. The cells do not materially change their position, 

 but each goes over into a different part of the body from that of 

 which it formerly made a part. Since, however, as in the 

 case of Lumbriculus, each cell may develop into a part of either 

 the anterior or posterior end of the new hydra, according to the 

 level at which the piece was cut off, we must account for the 

 one or the other change. What factors can we suppose bring 

 about this result } In the first place, however the piece is 



1 The specific substances that the new cells have brought with them from the 

 old parts may in some cases determine the kind of new organ rather than the rela- 

 tion of the new organ to the rest of the body (through, of course, its area of con- 

 tact with the old body). In this way a new eye may develop in the hermit crab 

 when the distal end of the old eye stalk is cut off, and not an eye but an appendage 

 when the stalk is cut off at its base. The development of a tail at the anterior 

 end of a posterior piece of the earthworm would be due to the specific character 

 of the new cells dominating the development. 



