60 GENETICS IN RELATION TO AGRICULTURE 
results from the longitudinal splitting of the original thread. Both parts 
of the thread are quantitatively and qualitatively equal, for, by the 
splitting of all the chromomeres both of the 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 segments 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 mechanism 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 diagrammed 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 # they have moved nearly to the opposite poles of the spindle 
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 completely lost all appearance of their identity, the 
chromatin material is distributed throughout the nucleus as in the origi- 
-nal cell shown at A, 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 essen- 
tial fact of mitosis which is that the chromatin material of the cell is 
converted into a thread which splits throughout its entire length into two 
halves so that the daughter nuclei receive exactly equivalent portions 
of chromatin material. This precise division of the chromatin 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 number of chromosomes contrib- 
uted 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 num- 
ber of chromosomes to one-half that characteristic of the somatic cells. 
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