64 



FUNDAMENTALS OF CYTOLOGY 



telophase has advanced so far. The stage between two mitoses occumng 

 in rapid succession is called the interphase. 



The division cycle of the chromonemata in somatic mitosis may be 

 summarized as follows for a single chromosome. In the metabolic stage 

 the chromosome is represented by two chromonemata which rank as 

 chromatids. In the early prophase these appear as a double spiral 

 thread. As the prophase advances the two become less closely asso- 

 ciated, and by the time the metaphase is reached each of them has 

 divided into two half-chromatids, making four chromonemata in the 

 whole metaphase chromosome. In the anaphase the two chromatids 

 move apart toward opposite poles. Each is now an independent daughter 

 chromosome, and its two chromonemata are now chromatids. These 

 chromonemata represent the chromosome through the telophase and 

 the ensuing metaboljc stage. Thus a chromonema becomes visibly 



ABC abc 



ABCcibc 



Fig. 44. — Diagram illustrating the equational character of somatic initosis. 



double slightly over one mitotic cycle in advance of the time at which 

 the halves are to separate. There are reasons for believing that the 

 threads are doubled submicroscopically before any doubleness is seen 

 and, further, that the chromosomes, particularly large ones, may even 

 be more highly compound in terms of visible chromonemata than indi- 

 cated in these paragraphs. The foregoing will serve as a convenient 

 provisional disposition of the matter until some alternative interpretation 

 has become better established. Chromosome structure will be discussed 

 further in Chap. VII. 



Finally, the significance of the mitotic form of nuclear division may 

 be emphasized. At the close of a typical mitosis there are two nuclei 

 that are quantitatively and qualitativelj^ similar to each other and to 

 the nucleus from which they arose. The qualitative aspect is of special 

 significance. The nucleus is not merely a homogeneous mass of some 

 protein or other substance, but an intricately organized system of mate- 

 rials of many kinds with definite chemical and spatial relations. The 

 chromonemata contain a series of special constituents essential to normal 

 development, and in mitosis these constituents, after being doubled, 



