THE PHYSICAL BASIS OF MENDELISM 565 



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 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 

 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 



