NUCLEAR AND CELL-DIVISION. 



Fig. 29- 



Figs 27-29. Mitotic cell-division of fertilized whitefish eggs Coregonus albus. 

 Fig. 27, Metakinesis stage; Fig. 28, diaster; Fig. 29, late stage of dispirem, the 

 cell -protoplasm almost divided. 



nucleus. This skein of threads is known as the spirem or mother 

 skein, and may appear as a single thread, which breaks up into a 

 definite number of segments, or the segments may appear as such 

 when the skein is forming. At first the threads are coarse and often 

 somewhat irregular, staining much more deeply than the limn 

 network. The separate segments of chromatin are known as 

 chromosomes (Waldeyer, 88). They appear, as a rule, in the form 

 of rods varying in length and thickness, and staining very deeply, 

 and often bent into characteristic U-shaped loops. The bent portion 

 of each loop is called its crown. " Every species of plant or ani- 

 mal has a fixed and characteristic number of chromosomes, which 

 regularly recurs in the division of all its cells ; and in all forms 

 arising by sexual reproduction the number is even" (Wilson, 96). 

 In man the number of chromosomes is given as sixteen by Barde- 

 leben (92) and Wilson (96), and as twenty-four by Flemming (98). 

 During the formation of the spirem the nuclear membrane, as a 

 rule, disappears. The nucleolus is also lost sight of, although the 

 manner of its disappearance can not be definitely stated. The net- 

 knots are no doubt taken up by the chromosomes. The chromo- 



