IV] 



STRUCTURE OF THE CELL 



173 



The following is a brief epitome of the visible changes undergone 

 by a typical cell, leading up to the act of segmentation, and con- 

 stituting the phenomenon of mitosis or caryokinetic division. In 

 the egg of a sea-urchin, we see with almost diagrammatic com- 

 pleteness what is set forth here*. 



1. The chromatin, which to begin with was distributed in 

 granules on the otherwise achromatic reticulum (Fig. 42), concen- 

 trates to form a skein or sfireme, which may be a continuous 

 thread from the first (Figs. 43, 44), or from the first segmented. 

 In any case it divides transversely sooner or later into a number 

 of chromosomes (Fig. 45), which as a rule have the shape of little 



Fig. 44. 



rods, straight or curved, often bent into a V, but which may 

 also be ovoid, or round, or even annular. Certain deeply staining 

 masses, the nucleoli, which may be present in the resting nucleus. 

 do not take part in the process of chromosome formation ; they 

 are either cast out of the nucleus and are dissolved in the cyto- 

 plasm, or fade away in situ. 



2. Meanwhile, the deeply staining granule (here extra - 

 nuclear), known as the centrosome, has divided in two. The two 

 resulting granules travel to opposite poles of the nucleus, and 



* My description and diagrams (Figs 42 — 51) are based on those of Professor 

 E. B. WUson. 



