10 



HISTOLOGY. 



apart, the diplosome thus separating into two centrosomes, and the centro- 

 sphere becoming divided into two spheres, each of which contains a centro- 

 some, Fig. 9. 



The nucleus meanwhile enlarges and its chromatin stains much more 

 deeply. The branching portions of the chromatin network are withdrawn, 

 so that instead of a net, the entire chromatic material forms one convoluted 

 thread, a monospireme, as this mitotic figure is called. The thread is at 

 first more closely coiled than it is later. It divides transversely into a 

 definite number of segments, called chromosomes. These bodies may be 

 spherical or rod-like, but generally they are U- or V-shaped. The apices 

 of all the V's may at first point toward the centrosome with their free ends 

 directed away from it as shown in the diagram, Fig. 9. Instead of being 

 arranged in the orderly manner of the diagram, however, the chromosomes 



Central spindle. 



Chromosomes. 



Centrosome. 





FIG. 9. EARLY PROPHASE: MONO- 

 SPIREME. 



FIG. 10. LATER PROPHASE: MONO- 

 SPIREME. 



are so massed that they can scarcely be counted. This is shown in Fig. 15, 

 representing mitoses in the salamander. In man they are even harder to 

 count and have been estimated both as sixteen and twenty-four. This 

 i of importance, since in any one species the number of chromosomes 

 is believed to be constant for all the cells except the sexual cells. Certain 

 worms in which the chromosomes are only two or four in number and hence 

 can be followed with certainty, have furnished the strongest evidence 

 for this. Except in the sexual cells, the number of chromosomes is always 

 even. Since it has been found that the same number of chromosomes 

 which entered into the formation of the chromatin network of the resting 

 nucleus, will emerge from that net preceding mitosis, the suggestion is 

 made that the chromosomes retain their individuality in the qxiiescent 

 nucleus. They are regarded as disguised by numerous branches. In 

 the prophase of mitosis the chromatin in many cases does not form a 



