THE BIOLOGICAL I'.ACKGROUND OF GENETICS 



195 



Now the real separation phase of division begins. The twin com- 

 ponents of each chromosome seem to be pulled apart, possibly by the 

 fibers attached to them, and one twin migrates to one pole, the other 

 twin to the other (Fig. 43, 77). Slowly the various chromosome twins 

 approach opposite poles and finally clus- 

 ter closely about the latter. At this 

 point the cell wall constricts at the equa- 

 tor and soon divides the single cell into 

 twin daughter cells (Fig. 43, 7"). Not 

 only are the chromosomes divided me- 

 ticulously into equal lots, but all the 

 cytoplasmic parts of the cell are so 

 divided that each daughter cell comes to 

 have exactly the same detailed organiza- 

 tion as had the mother cell from which 

 they were derived. The period during 

 which chromosome halves migrate to the 

 poles of the spindle is called the anaphase, 

 while that during which the chromosomes 

 lose their density and reform the chroma- 

 tin network with which we started is 

 called the telophase. 



By means of this elaborate routine 

 of division the exact specific organization 

 of the cell is maintained, at least in the 

 germ track. In this process of mitosis, 

 we have a view of the mechanics of hered- 

 ity, which, if uninterrupted, would main- 

 tain perfect constancy of type for all time 

 unless a new order of environment were 



to alter the expression of a fixed hered- -Diagram of the 



. ii- middle phases of mitosis. E, met- 



itary complex, or some other mechanism aphase . F> G> ear]ier and ]ater 



promoting diversity were to intervene. anaphases. (From Wilson.) 



DIFFERENTIATION 



Not merely do cells multiply in the process of embryonic develop- 

 ment, but at every step differentiation of cells for the performance of 

 different specialized functions is going on. The mechanism of differen- 

 tiation is at present very incompletely understood, and what we do 

 know is of a technical nature and difficult to present without a back- 



