DEVELOPMENT OF THE GERM-CELLS 131 



D to G. In one of these, F, the unaltered germ-plasm 

 is supposed to be carried on. The third division leads 

 to Stage 4, with eight cells marked H to O, L being the 

 carrier of the germ-plasm. Finally, the fourth and last 

 division leads to the ultimate Stage 5, with sixteen 

 body-cells indicated by P to W, the two cells of each 

 pair being marked by the same letter. We must also 

 suppose that the minute mass of germ-plasm, a, grows 

 and separates as a germ-cell or germ-cells from either L 

 or one or more of the somatic cells into which the latter 

 divides. The four germ-cells of the adult Metazoon 

 are then produced by division. These germ-cells are, 

 therefore, similar to that which started development ; 

 they are, in fact, a piece of it, which has grown without 

 undergoing any essential alteration. The four germ- 

 cells will, therefore, tend to produce offspring resembling 

 their parents. It must be borne in mind, however, that 

 in actual development the precursors of future germ-cells 

 become recognizable as a definite group at a far earlier 

 stage than that shown in the diagram. In certain species, 

 e. g. Cyclops and Ascaris, the germ-antecedent has been 

 traced at almost the very beginning of development. In 

 Ascaris megalocephala, Boveri has shown that one of the 

 two cells formed by the first division of the egg con- 

 tains the germ-antecedents together with many other 

 potentialities. These latter are gradually told off in the 

 succeeding divisions, until, at the sixth, a single cell out of 

 the sixty-four into which the egg is then divided is the 

 precursor of the future germ-cells and germ-duct, but 

 bears no other potentiality. 1 



If, however, some of the somatic cells become modified 

 from that nature which was predetermined in the germ- 

 plasm of the ovum, there is no way in which the heredi- 

 tary transmission of such modifications can be explained by 

 the hypothesis of the continuity of the germ-plasm ; for 

 it does not include any means by which the effects could 

 be conveyed to the germ-cells, or, if conveyed, could 

 produce in them changes such as would predetermine 



1 Kupffer's Festschrift, Jena, 1899, p. 383, Die Entwickelung von 

 Ascaris megalocephala. 



K 2 



