tAP. xii CELL-DIVISION 561 



the rays are becoming attached to the chromosomes. Z>, perfected nuclear 

 spindle with the two centrospheres at the poles (cspti) and the eight chromo- 

 somes (chrs) in the equator of the spindle, all now longitudinally split. E, 

 daughter chromosomes diverging from one another, but still united by filaments ; 

 the centrosomes (cs) are already doubled for the next division. F, daughter 

 chromosomes, quite separated from one another, are already beginning to give 

 off processes ; the cell-body is beginning to be constricted. G, end of the 

 process of division ; two daughter-cells (tz) each with nuclear network (tk) 

 and centrospheres (csph) as in A. (From Weismann's Evolution Theory, 

 adapted from E. B. Wilson.) 



Cell-division. The precise changes which take place 

 during the fission of a cell are, like the structure of the cell 

 itself, subject to considerable variation. We will consider 

 what may probably be taken as a typical case (Fig. 146). 



First of all, the chromatin grains (chr) came together so 

 as to form a loose coil or skein (B). The centrosome, and 

 then the centrosphere (csph\ divides, and from the latter 

 fine protoplasmic filaments are seen to radiate ; the products 

 of its division gradually separate from one another (c), 

 ultimately passing to opposite poles of the nucleus (D), 

 delicate threads extending from one to another in the form 

 of a spindle (ksp). At the same time the nuclear mem- 

 brane and the nucleoli usually disappear, and the chromatin 

 skein divides into a number of separate pieces of equal 

 length called chromosomes (c, chrs\ the number of which 

 appears to be constant in any given species of animal or 

 plant, although it may vary in different species from two 

 to a hundred and sixty-eight or more. 



The chromosomes become arranged in the equatorial 

 plane of the spindle and each of them splits along its whole 

 length, so as to form two parallel rods or loops in close 

 contact with one another (D), and arranged in a radiating 

 manner so as to present a star-like figure when the cell 

 is viewed in the direction of the long axis of the spindle : in 

 this way the number of chromosomes is doubled, each one 

 being now represented by a pair. Everything is now ready 



PRACT. ZOOL. O O 



