Nuclei as Carriers of Hereditary Characters 107 



haploid generation in a phanerogamic plant or in a vertebrate 

 animal. In Angiosperms this is actually represented only by the 

 short developmental stages which extend from the pollen mother- 

 cells to the sperm-nucleus of the pollen-tube, and from the embryo- 

 sac mother-cell to the egg and the endosperm tissue. The embryo- 

 sac remains enclosed in the diploid ovule, and within this from the 

 fertilised egg is formed the embryo which introduces the new diploid 

 generation. On the full development of the diploid embryo of the 

 next generation, the diploid ovule of the preceding diploid genera- 

 tion is separated from the latter as a ripe seed. The uninitiated 

 sees in the more highly organised plants only a succession of diploid 

 generations. Similarly all the higher animals appear to us as in- 

 dependent organisms with diploid nuclei only. The haploid genera- 

 tion is confined in them to the cells produced as the result of the 

 reduction division of the gonotokonts ; the development of these 

 is completed with the homotypic stage of division which succeeds the 

 reduction division and produces the sexual products. 



The constancy of the numbers in which the chromosomes 

 separate themselves from the nuclear network during division gave 

 rise to the conception that, in a certain degree, chromosomes possess 

 individuality. Indeed the most careful investigations 1 have shown 

 that the segments of the nuclear network, which separate from one 

 another and condense so as to produce chromosomes for a new 

 division, correspond to the segments produced from the chromo- 

 somes of the preceding division. The behaviour of such nuclei as 

 possess chromosomes of unequal size affords confirmatory evidence 

 of the permanence of individual chromosomes in corresponding 

 sections of an apparently uniform nuclear network. Moreover at 

 each stage in division chromosomes with the same differences in size 

 reappear. Other cases are known in which thicker portions occur in 

 the substance of the resting nucleus, and these agree in number 

 with the chromosomes. In this network, therefore, the individual 

 chromosomes must have retained their original position. But the 

 chromosomes cannot be regarded as the ultimate hereditary units in 

 the nuclei, as their number is too small. Moreover, related species 

 not infrequently show a difference in the number of their chromo- 

 somes, whereas the number of hereditary units must approximately 

 agree. We thus picture to ourselves the carriers of hereditary 

 characters as enclosed in the chromosomes ; the transmitted fixed 

 number of chromosomes is for us only the visible expression of the 

 conception that the number of hereditary units which the chromo- 

 somes carry must be also constant. The ultimate hereditary units 



1 Particularly those of V. Gr^goiie and his pupils. 



