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. 2S B). 



The moment that the spermatozoon touches 

 the surface of the egg the latter throws out at 

 the point touched a prominence, or reception 



