viii RECRESCENCE FROM MINUTE FRAGMENTS 395 



govern them, when the organism is conceived as the 

 expression of a sum of forces which mutually supplement 

 one another, which govern the particles and restore their 

 equilibrium when it is destroyed — restore equilibrium in the 

 sense of reconstituting the previously existing whole. 



That a tendency to develop into the whole by growth is 

 impressed upon the several particles of the organism by 

 heredity is best shown by the fact that many multicellular 

 plants and animals can be subdivided almost into their con- 

 stituent cells with the result that each portion grows again 

 into an entire organism. Since Trembley's time it has been 

 known that the smallest parts into wdiich our common Hydra 

 is divided become new entire individuals. The question, 

 however, arises whether such fragments must not consist 

 of at least two connected cells, an ectoderm - cell and an 

 endoderm-cell, if recrescence is to ensue ^ — because the ecto- 

 derm and the endoderm even in these lower animals respec- 

 tively have special characters established by heredity. In 

 any case, recrescence from a single cell must be possible in 

 those lower organisms which consist of simple colonies. It 

 takes place in this way even in higher plants. A very 

 remarkable instance of this is recorded by Vochting:" "A 

 piece from the middle of the surface of a vigorous thallus of 

 Lunularia vulgaris was cut up with a sharp knife on a 

 smooth plate of cork until the fragments were so small as to 

 form a coarse-grained pulp. The largest fragments were 

 about half a cubic millimetre in size, while the smallest were 

 considerably smaller. This pulp was spread out on moist 

 sand and protected as much as possible from external dis- 

 turbing agents. After some time young sprouts showed 



^ According to Th. W. Engelmaun this is in fact the case, Cf. Zool. Anz. I. 

 p. 77, 1878. 



- H. Vdchting, Ueher die Regeneration der Marchantien, Pringsheim's 

 Jahrhucher fur wissensch. Botanik, Bd. jxvi. heft 3, jip. 15, 16, Berlin, 

 1885. 



