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' 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. Grégoire and his pupils. 
