FOUNDATION OP A THEORY OF HEREDITY. 197 



different from that which is contained in the germ-cell, and we 

 have no right to assume that any of them can form germ-cells until 

 it is proved that somatic idioplasm is capable of undergoing re- 

 transformation into germ-idioplasm. 



The same argument applies to the cells of the embryo itself, and 

 it therefore follows that those instances of early separation of 

 sexual from somatic cells, upon which I have often insisted as 

 indicating the continuity of the germ-plasm, do not now appear to 

 be of such conclusive importance as at the time when we were not 

 sure about the localization of the idioplasm in the nuclei. In the 

 great majority of cases the germ-cells are not separated at the 

 beginning of embryonic development, but only in some one of the 

 later stages. A single exception is found in the pole-cells (' Pol- 

 zellen ') of Diptera, as was shown many years ago by Robin J and 

 myself 2 . These are the first cells formed in the egg, and accord- 

 ing to the later observations of Metschnikoff 3 and Balbiani 4 , they 

 become the sexual glands of the embryo. Here therefore the 

 germ -plasm maintains a true unbroken continuity. The nucleus 

 of the egg-cell directly gives rise to the nuclei of the pole-cells, 

 and there is every reason to believe that the latter receive un- 

 changed a portion of the idioplasm of the former, and with it 

 the tendencies of heredity. But in all other cases the germ-cells 

 arise by division from some of the later embryonic cells, and as 

 these belong to a more advanced ontogenetic stage in the de- 

 velopment of the idioplasm, we can only conclude that continuity 

 is maintained, by assuming (as I do) that a small part of the germ- 

 plasm persists unchanged during the division of the segmentation 

 nucleus and remains mixed with the idioplasm of a certain series of 

 cells, and that the formation of true germ-cells is brought about at 

 a certain point in the series by the appearance of cells in which the 

 germ-plasm becomes predominant. But if we accept this hypo- 

 thesis it does not make any difference, theoretically, whether the 

 germ-plasm becomes predominant in the third, tenth, hundredth, 

 or millionth generation of cells. It therefore follows that cases 

 of early separation of the germ-cells afford no proof of a direct 



1 'Compt. rend.' Tom. LIV. p. 150. 



2 ' Entwicklung der Dipteren.' Leipzig, 1864. 



3 'Zeitschr. f. wiss. Zool.' Bd. XVI. p. 389 (1866). 



4 ' Compt. rend.' Nov. 13, 1882. 



