74 ON HEREDITY. [II. 



ductive 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 eg^ into the embryo, 

 that is the segmentation of the ^g%^ 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 -liey are taken into 

 the interior of the animal and form its reproductive organs. 

 Again, in certain small freshwater Crustacea [Daphnidae) the 

 future reproductive cells become distinct at a very early 

 period, although not quite at the beginning of segmentation, 

 i. e. when the egg has divided into not more than thirty seg- 

 ments. Here also the cells which are separated early form the 

 reproductive organs of the animal. The separation of the re- 

 productive cells from those of the body takes place at a still 

 later period, viz. at the close of segmentation, in Sagitta — a 

 pelagic free-swimming form. In Vertebrata they do not be- 

 come distinct from the other cells of the body until the embryo 

 is completely formed. Thus, as their development shows, 

 a marked antithesis exists between the substance of the un- 

 dying reproductive cells and that of the perishable body-cells. 

 We cannot explain this fact except by the supposition that each 

 reproductive cell potentially contains two kinds of substance, 

 which at a variable time after the commencement of embryonic 

 development, separate from one another, and finally produce 

 two sharply contrasted groups of cells. 



It is evidently unimportant, as regards the question of here- 

 dity, whether this separation takes place early or late, inasmuch 

 as the molecular constitution of the reproductive substance is 

 determined before the beginning of development. In order to 

 understand the growth and multiplication of cells, it must be 

 conceded that all protoplasmic molecules possess the power of 

 growing, that is of assimilating food, and of increasing by means 

 of division. In the same manner the molecules of the repro- 

 ductive protoplasm, when well nourished, grow and increase 

 without altering their peculiar nature, and without modifying 

 the hereditary tendencies derived from the parents. It is 

 therefore quite conceivable that the reproductive cells might 

 separate from the somatic cells much later than in the examples 

 mentioned above, without changing the hereditary tendencies 

 of which they are the bearers. There may be in fact cases in 



