ON HEEEDITY. 75 



And this theoretical deduction is confirmed by observation, for from 

 the egg- of a Medusa, produced by the budding of a Polype, a 

 Polype, in the first instance, and not a Medusa arises. Here the 

 molecules of the reproductive substance first formed part of the 

 Polype, and later, part of the Medusa bud, and, although they 

 separated from the somatic cells in the bud, they nevertheless 

 always retain the tendency to develope into a Polype. 



We thus find that the reproduction of multicellular organisms is 

 essentially similar to the corresponding process in unicellular forms ; 

 for it consists in the continual division of the reproductive cell ; 

 the only difference being that in the former case the reproductive 

 cell does not form the whole individual, for the latter is composed 

 of the millions of somatic cells by which the reproductive cell is 

 surrounded. The question, 'How can a single reproductive cell 

 contain the germ of a complete and highly complex individual ? ' 

 must therefore be re-stated more precisely in the following form, 

 ' How can the substance of the reproductive cells potentially con- 

 tain the somatic substance with all its characteristic properties?' 



The problem which this question suggests, becomes clearer when 

 we employ it for the explanation of a definite instance, such as the 

 origin of multicellular from unicellular animals. There can be 

 no doubt that the former have originated from the latter, and that 

 the physiological principle upon which such an origin depended, is 

 the principle of division of labour. In the course of the phyletic 

 development of the organized world, it must have happened that 

 certain unicellular individuals did not separate from one another 

 immediately after division, but lived together, at first as equivalent 

 elements, each of which retained all the animal functions, including 

 that of reproduction. The Magosphaera planula of Ha'ekel proves that 

 such perfectly homogeneous cell-colonies exist 1 , even at the present 

 day. Division of labour would produce a differentiation of the single 

 cells in such a colony : thus certain cells would be set apart for ob- 

 taining food and for locomotion, while certain other cells would be 

 exclusively reproductive. In this way colonies consisting of somatic 

 and of reproductive cells must have arisen, and among these for 



1 It is doubtful whether Magosphaera should be looked upon as a mature form ; 

 but nothing hinders us from believing that species have lived, and are still living, in 

 which the ciliated sphere has held together until the encystment, that is the re- 

 production, of the constituent single cells. 



