PROTOPLASM AND THE CELL 63 



surrounding protoplasm has produced some rays radiating from each 

 centriole. These two bodies are now known as asters because of their 

 starlike appearance. The two asters taken together are called the 

 aniphiaster. The nucleolus disappears and the chromatin which ap- 

 parently up until this time has been somewhat dispersed through the 

 nuclear substance in the form of granules, becomes organized into 

 long, possibly tangled, chromatin fibers or threads. (Some hold that 

 the chromatin retains its linear arrangement from one cell genera- 

 tion to the next.) Each consists of a double linear series of chromatin 

 bodies like two chains bound closely together to form the thread. 

 A single one of these two series in the thread is known as a chromo- 

 nema (pi., chromonemata) . The chromatin bodies comprising the 

 chromonemata are often called chromomeres (Fig. 21). 



In the middle prophase the centrioles have migrated still farther 

 from each other and the splindle fibers between the centrioles as well 

 as the astral rays around them have become well established. Ac- 

 cording to modern explanation, each of the chromatin threads now 

 shortens and condenses to become a chromosome. There is a constant 

 number of these in the cells of each species. During the above 

 changes, the nuclear membrane begins to degenerate. In the late 

 prophase, the centrioles have reached the polar positions on opposite 

 sides of the nucleus. The spindle extends between the two asters and 

 the chromosomes become arranged on the spindle fibers in an orderly 

 fashion midway between the centrioles to form the equatorial plate. 

 It has been reported that, in certain cells at least, the prophase stage 

 requires about eight minutes. The nuclear membrane now has al- 

 most completely disappeared. 



The metaphase follows with no interruption. The chromosomes, 

 still arranged in the equatorial plate, now each split longitudinally, 

 placing one chromonema in each part. The characteristic feature of 

 the metaphase is this splitting. Following this stage each "of the new 

 chromosomes, resulting from this splitting or division, migrates along 

 the spindle fibers toward its respective centriole, or pole. This period 

 is the anaphase. These " half -chromosomes " each soon come to have 

 two chromonemata, and they carry the chromatin material of the new 

 cells which result from the ensuing division. The explanation of the 

 movement of these chromosomes from the equatorial plate out to the 

 poles is not entirely forthcoming, although there is general belief that 

 the spindle fibers, with which they are associated, are involved in the 

 phenomenon. 



