Physical Basis of Heredity and Evolution 31 



one case, the gamete becomes larger and larger, its power 

 of movement diminishing at the same time, until at last 

 it becomes a very large and entirely passive cell. In the 

 other case, the gamete retains its small size and activity. 

 These two very dissimilar gametes are the egg and the 

 sperm, easily distinguishable female and male cells (Fig. 4). 

 The essential difference thus brought about is the great in- 



FIG. 4. -A single large egg and numerous small sperms of rockweed (Fucus) 



crease in the bulk of the gamete that becomes the egg, 

 but the constant feature, which is not changed, is the chro- 

 matin, the increase in bulk being due to an increase of 

 cytoplasm. This constancy of the chromatin, coupled with 

 the known fact that the two gametes contribute alike to 

 their progeny, indicates that the chromatin is the essential 

 material in heredity. Since both cytoplasm and nuclei are 

 involved in the sexual fusion, it may be claimed that the 



