THE CELLULAR BASIS 109 



living substances which are of great signifi- 

 cance in heredity. Usually only the head of 

 the spermatozoon enters the egg (Figs. 4-7) 

 and this consists almost entirely of nuclear 

 material which has a strong chemical affinity 

 for certain dyes, and hence is called chromatin 

 (Fig. 23 A and 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 a sub- 

 stance which is not easily stained by dyes and 

 hence is called achromatin (Fig. 23 D and E) . 

 The chromatin then becomes scattered through 

 each nucleus in the form of granules or threads 

 which are embedded in the achromatin; this is 

 the condition of a typical "resting" 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. 23 B ) . 



The moment that the spermatozoon touches 

 the surface of the egg the latter throws out at 

 the point touched a prominence, or reception 



