THE CELLULAR BASIS 115 



cance in heredity. Usually only the head of 

 the spermatozoon enters the egg (Figs. 3, 4) 

 and this consists almost entirely of nuclear 

 material which has a strong chemical affinity 

 for certain dyes, and hence is called chromatin 

 (Fig. 4 D-H, 23 A-B) ; when the egg has ma- 

 tured and is ready to be fertilized its nucleus 

 also consists of a small mass of chromatin 

 (Fig. 23 C) . Both of these condensed chro- 

 matic nuclei then grow in size and become less 

 chromatic by absorbing from the egg sl sub- 

 stance which is not easily stained by dyes and 

 hence is called achromatin (Figs. 4 I-L, 23 

 D-E) . The chromatin then becomes scattered 

 through each nucleus in the form of granules 

 or threads which are embedded in the achro- 

 matin; this is the condition of a typical "rest- 

 ing" nucleus. The spermatozoon also brings 

 into the egg a centrosome or division center, 

 around which an aster appears consisting of 

 radiating lines in the protoplasm of the egg 

 (Fig. 4F-J). 



The moment that the spermatozoon touches 

 the surface of the egg the latter throws out at 



