60 



APPLIED BIOLOGY 



new nucleus containing 

 the chromosomes in 

 each of the two cells 

 formed by the division. 

 In warm spring 

 weather the first di- 

 vision is usually com- 

 pleted within two hours 

 after the egg-cells are 

 laid in the water, and 

 the egg is then in the 

 two-cell stage (Fig. 23). 

 Each cell appears ex- 

 actly like the original 

 egg-cell, but is one half 

 the size. In another 

 hour each cell of the 

 two-cell stage will have 

 gone through the same 

 processes of division, 



FIG. 22. Diagrams illustrating division of a and the four-cell stage 

 cell. A, resting cell with nucleus (n) and -11 i rpaP hf>H 

 centrosome (c). B, preparing to divide, 



two asters (a) near nucleus, each with a Such divisions of Cells 

 centrosome, chromatin becoming massed 

 into chromosomes. C, two asters have 

 formed a spindle with chromosomes (ch) in 

 center. D, each chromosome divided and 

 two halves being moved toward the asters. . , 



E, chromosomes forming the two new eight, Sixteen, and 

 nuclei, and cell-body beginning to divide, 



F, division complete, two-cell stage, each 

 cell has the same structure as the one cell 

 in A. cw, new cell-wall. Compare with 



dividins in Figs ' 40> 41- 



O CCUr about every hour ; 



. J 



and SO, in SUCCCSSlOn, 



stages of two four 



thirty-two Cells are 



t i /!? oo\ T 4 



^Tmed (S Ig. 23). Later 



some cells may get 



slower in division ' and 



hence the stages only 



approximate 64, 128, 256 and more cells, multiplying by two 

 when all the cells have divided. Usually within two or 

 three days the egg has reached a stage consisting of a spherical 



