28 I The Process of Evolution 



with remarkable autonomy. Sex chromosomes may appear as pre- 

 cocious or tardy in comparison with the autosomes. Speciahzed 

 chromosomes may be confined to the germ hne and become ehmi- 

 nated in later divisions. In the fungus gnat Sciara a monopolar 

 spindle is formed at one division, and one group of chromosomes 

 moves to the "nonpolar" end. Occasionally in animals the nuclear 

 envelope does not disappear, and chromosome division takes place 

 within the membrane, which eventually is pinched in two. As more 

 work is carried out on the little-known invertebrates, algae, and 

 fungi, other examples of unusual behavior undoubtedly will be 

 found. Indeed, when proper perspective is reached as a result of 

 systematic study, the higher plant-vertebrate mechanisms may 

 seem unusual. 



When the chromosomes have reached the poles of the spindle, a 

 new nuclear envelope, which may arise from one of the membrane 

 systems of the cytoplasm, is formed about each group of daughters. 

 During this stage, telophase, animal cells usually divide by furrow- 

 ing and plant cells by cell-plate formation. (A new cell wall parti- 

 tions the old cell.) With this formation of two daughter cells, the 

 process of cell division ends. Thus two cells, each with the same 

 genotype, have been produced as a result of equational division of 

 the chromosomes (mitosis) and division of the cytoplasm, during 

 which the cytoplasmic organelles are apportioned roughly equally. 



During the course of mitosis, the chromosomes (Fig. 2.3) go 

 through an interesting and important series of changes. If a prophase 

 chromosome is compared with an anaphase chromosome, striking 

 differences are seen. The anaphase chromosome is not only easily 

 visible and stainable but fatter and much shorter. By use of appro- 

 priate treatments, it can be shown that the anaphase chromosome 

 is in the form of a tight spiral, the gyres ( turns of the coil ) of which 

 behave as if they were invested with a stainable substance usually 

 called matrix. The basic thread or chromonema of an anaphase 

 chromosome often can be seen to be coiled in a fine series of gyres 

 called minor coils. The easily seen major coils are imposed upon 

 these by whatever process causes the shortening of the chromo- 

 somes. When there is more than one chromatid, as in metaphase, 

 these may be twisted about one another. 



Mitosis provides for the equational division of the chromosomes 

 so that, barring a mutational event, each daughter cell receives the 

 same genetic information. In many instances, as cells become spe- 

 cialized in form and function, division of the chromosomes may 

 occur without division of the nucleus or of the cell. The result is 

 cells with more than one nucleus or nuclei with more than the zygotic 



