THE PHYSICAL BASIS OF MENDELISM 405 
threads come to possess all of the individual elements of the original 
spireme thread. Following the splitting of the chromomeres and the 
formation of a double spireme, the spireme-thread contracts and seg- 
ments transversely forming four double chromosomes, the number 
characteristic of the cells of this individual. This is the stage shown 
at C where also is shown the origin of the spindle, a part of the mechan- 
ism in mitosis. The chromosomes now still further contract until 
they assume their characteristic shapes and sizes. They next appear 
in an equatorial position on the spindle as shown at D, where the two 
pairs of double chromosomes, one larger and one smaller, are dia- 
grammed and the nucleolus, the large black body of the previous steps, 
is shown cast out and degenerating. The daughter chromosomes of 
each pair now separate from each other until at E they have moved 
nearly to the opposite poles of the spindles and are beginning to fray 
out and seemingly to lose their identity. At this stage actual division 
of the cell body has begun. Finally at F, the chromosomes have com- 
pletely lost all appearance of their identity, the chromatin material 
is distributed thruout the nucleus as in the original cell shown at 4, 
and the nucleolus has been reformed in each nucleus. Division of the 
cell-body has resulted in two daughter cells, each of which, so far as 
chromomeres are concerned, contains exactly the same chromatin 
elements as the original cell. 
There are many variations in this process particularly in the order 
of occurrence of the steps, but these variations in nowise modify the 
essential fact of mitosis which is that the chromatin material of the 
cell is converted into a thread which splits thruout its entire length 
into two halves so that the daughter nuclei receive exactly equivalent 
portions of chromatin material. This precise division of the chro- 
matin is brought about by a division of each chromomere so that not 
only do the daughter nuclei receive equivalent portions of chromatin 
but these portions are also equivalent qualitatively to the entire 
chromatin content of the mother cell. By this method then each of 
the cells of the body finally comes to possess not only the whole num- 
ber of chromosomes contributed by the two parents, but also the 
entire set of chromatin elements which it received from them. The 
extreme care with which the cell mechanism partitions the chromatin 
material in each successive cell division is in itself eloquent testimony 
of the fundamental importance of this material. 
The production of germ cells.—In the production of germ cells a 
different set of phenomena occur which result in a reduction of this 
