104 



The Nature of Biological Diversity 



of the metazoan chromosome? A plausihle answer is that their role is 

 primarily to regulate the function of the DNA — a regulation which 

 becomes of critical importance in all organisms composed of many 

 diverse types of cells all equipped with the same set of genes. 





FIG. 3. Electron micrograph of pigmented retina of a C57 brown mouse 18.5 days 

 in utero. Fixed in osmic acid, embedded in methacrylate, and stained with lead 

 subacetate. Four stages in melanosome development are shown. Note the longitu- 

 dinal (4) and cross sectional (3) cuts through the melanosomes. (Courtesy of Dr. 

 Frank H. Moyer.) 



Direct cytochemical investigations of chromosomes is one promis- 

 ing area of investigation. Recently Bloch and Hew ( 1960 ) have shown, 

 in a cytochemical investigation of snail spermatogenesis, that the 

 spermatocyte chromosomes contain histones with staining properties 

 indistinguishable from other somatic tissues. As the differentiation 

 of sperm proceeds, the somatic histone (lysine-rich ) is replaced by 



