10 THE BREEDING OF ANIMALS 



In the ox and in man the number is sixteen. The fact 

 that the number of chromosomes is even in all species is 

 due to the fact that during the processes of fertilization 

 one-half the chromosomes are derived from the female 

 and one-half from the male parent. 



After the breaking up of the spireme thread into a 

 definite number of chromosomes, there is formed in the 

 cell the so-called amphiaster. The development and 

 activities of this interesting structure seem to be for the 

 purpose of arranging the chromosomes in position for 

 division. All the processes concerned in the prophases 

 are preparatory to the final division, and ultimate dis- 

 tribution of chromatin to the new cell. 



11. Metaphase. Each chromosome now splits into 

 two exactly equal halves and the two new groups move 

 to opposite sides of the cell. The chromosomes divide 

 lengthwise, and by so doing there results an accurate 

 division of the chromatin into two precisely equivalent 

 portions. Each portion eventually becomes the nucleus 

 of one of the two new daughter cells which result from 

 this division. The most fundamentally important fact 

 about this division of the chromatin is that it is a qualita- 

 tive as well as a quantitative division. There is much evi- 

 dence to show that the spireme thread (and, therefore, 

 the chromosomes) is composed of granules or units 

 throughout its length, and each of these units represents 

 a definite character or set of characters in the individual. 

 It follows that when a lengthwise division occurs, these 

 units are divided and a portion of each is passed on to the 

 daughter cells (Fig. 2). 



The arrangement of the chromatin in the spireme, its 

 breaking up into chromosomes, and the splitting of the 

 latter into halves are all directed toward the accurate 



