Growth 325 



like nature of the nuclear substance gradually gives way 

 to elongate chromatic structures, or to a chromatic band, 

 the prophase of the nuclear division is well advanced. 



Later there appear well-defined nuclear segments, 

 termed chromosomes, these resulting apparently from the 

 aggregation and growth of chromatic substance in a cer- 

 tain area. Coincidently, the nucleolus is less stainable 

 and may show an apparent degeneration, foretelling its 

 final disappearance. The chromosomes thicken, the nu- 

 clear membrane disappears, and out of the mass of fibrous 

 protoplasmic elements now present, there is oriented first 

 a multipolar, and later a bipolar, spindle with the chro- 

 mosomes arranged as an equatorial plate. In this stage, 

 the metaphase, spindle fibers are attached to either side 

 of each chromosome, and a longitudinal split is apparent. 

 The halves of each chromosome separate and the " daugh- 

 ter ' groups move (anaphase) to opposite poles of the 

 cell, where the organization of the daughter nuclei pro- 

 ceeds (telophase). Here a new reticulum is ultimately 

 evolved and a nucleolus reappears, formed, doubtless, in 

 some manner from the nuclear material. Upon the re- 

 maining spindle fibers at the middle points thickenings 

 occur, and these gradually extend as a plate between the 

 two " daughter ' cells. Thus the cell-space and the 

 cytoplasm also are divided, and the cell division is com- 

 plete. 



Upon the reappearance of the chromosomes in every 

 successive mitotic vegetative division, the number of 

 these segments is constant; that is to say, there is a defi- 

 nite chromosome number for every species of plant, and 

 the same is true of animals. 



