74 ON HEREDITY. 



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 segments. Here also the cells which are 

 separated early form the reproductive organs of the animal. The 

 separation -of the reproductive cells from those of the body takes 

 place at a still later period, viz. at the close of segmentation, in 

 Sagilta a pelagic free-swimming form. In Vertebrata they do 

 not become 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 undying 

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

 cannot explain this fact except by the supposition that each re- 

 productive cell potentially contains two kinds of substance, which 

 at a variable time after the commencement of embryonic develop- 

 ment, separate from one another, and finally produce two sharply 

 contrasted groups of cells. 



It is evidently unimportant, as regards the question of heredity, 

 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 reproductive 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 re- 

 productive 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 I 

 be in fact cases in which such separation does not take place until 

 after the animal is completely formed, and others, as I believe that 

 I have shown *, in which it first arises one or more generations 

 later, viz. in the buds produced by the parent. Here also there is 

 no ground for the belief that the hereditary tendencies of the repro- 

 ductive molecules are in any way changed by the length of time 

 which elapses before their separation from the somatic molecules. 



1 Compare Weismann, ' Die Entstehung der Sexualzellen bei den Hydromedusen,' 

 . 1883. 



