ON HEREDITY. 73 



From these unicellular organisms we can to a certain extent 

 understand why the offspring 1 , being- in fact a part of its parents, 

 must therefore resemble the latter. The question as to why the 

 part should resemble the whole leads us to a new problem, that of 

 assimilation, which also awaits solution. It is, at any rate, an 

 undoubted fact that the organism possesses the power of taking- up 

 certain foreign substances, viz. food, and of converting them into 

 the substance of its own body. 



Among these unicellular organisms, heredity depends upon the 

 continuity of the individual during the continual increase of its 

 body by means of assimilation. 



But how is it with the multicellular organisms which do not 

 reproduce by means of simple division, and in which the whole 

 body of the parent does not pass over into the offspring ? 



In such animals sexual reproduction is the chief means of mul-~"~~ 

 tiplication. In no case has it always been completely wanting, 

 and in the majority of cases it is the only kind of reproduction. 



In these animals the power of reproduction is connected with 

 certain cells which, as germ-cells, may be contrasted with those 

 which form the rest of the body; for the former have a totally 

 different role to play; they are without significance for the life of 

 the individual 1 , and yet they alone possess the power of preserving 

 the species. Each of them can, under certain conditions, develope 

 into a complete organism of the same species as the parent, with 

 every individual peculiarity of the latter reproduced more or less 

 completely. How can such hereditary transmission of the characters 

 of the parent take place ? how can a single reproductive cell repro- 

 duce the whole body in all its details ? 



Such a question could be easily answered if we were only con- \ 

 cerned with the continuity of the substance of the reproductive cells 

 from one generation to another ; for this can be demonstrated 

 in some cases, and is very probable in all. In certain insects 

 the development of the egg into the embryo, that is the segmen- 

 tation of the egg, begins with the separation of a few small cells 

 from the main body of the egg. These are the reproductive cells, 

 and at a later period they are taken into the interior of the 

 animal and form its reproductive organs. Again, in certain 

 small freshwater Crustacea (Dapltnidae] the future reproductive 

 1 That is for the preservation of its life. 



