DEVELOPMENT OF THE GERM-CELLS rv 



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 ^g^ 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 ^g'g is then divided is the 

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

 bears no other potentiality.^ 



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 



' Kupfifer's Festschrift, Jena, 1899, p. 383, Die Entwichehing von 

 Ascaris meg aloe ephala. 



K 2 



