THE BEARERS OF THE HERITAGE 293 
In fact this whole complicated system of indirect division is regarded 
by most biologists as a mechanism for bringing about the precise 
halving of the chromosomes. 
The chromosomes of each group at the poles fmally fuse and two 
new nuclei, each similar-to the original one, are constructed (Fig. 44, 
gandk). In the meantime a division of the cell-body is in progress 
which, when completed, results in the formation of two complete 
new cells. 
As all living matter, if given suitable food, can convert it into living 
matter of its own kind, there is no difficulty in conceiving how the 
new cell or the chromatin material finally attains to the same bulk 
that was characteristic of the parent cell. In the case of the chro- 
matin, indeed, it seems that there is at times a precocious doubling 
of the ordinary amount of material before the actual division 
occurs. 
Chromosomes constant in number and appearance.—With some 
minor exceptions, to be noted later, which increase rather than detract 
from the significance of the facts, the chromosomes are always the 
same in number and appearance in all individuals of a given species 
of plants or animals. That is, every species has a fixed number which 
regularly recurs in all of its cell-divisions. Thus the ordinary cells 
of the rat, when preparing to divide, each display sixteen chromo- 
somes, the frog or the mouse, twenty-four, the lily twenty-four and 
the maw-worm of the horse only four. The chromosomes of different 
kinds of animals or plants may differ very much in appearance. In 
some they are spherical, in others rod-like, filamentous or perhaps of 
otherforms. In some organisms the chromosomes of the same nucleus 
may differ from one another in size, shape, and proportions, but if such 
differences appear at one division they appear at others, thus showing 
that in such cases the differences are constant from one generation to 
the next. 
Significance of the chromosomes.—The question naturally arises 
as to what is the significance of the chromosomes. Why is the accur- 
ate adjustment which we have noted for their division necessary ? 
The very existence of an elaborate mechanism so admirably adapted 
to their precise halving, predisposes one toward the belief that the 
chromosomes have an important function which necessitates the 
retention of their individuality and their equal division. Many biolo- 
gists accept this along with other evidences as indicating that in 
chromatin we have a substance which is not the same throughout, that 
