266 EVOLUTION AND ANIMAL LIFE 



in nowise affects the fundamental identity of the two. In the 

 maturation period the number of chromosomes in the nuclei 

 of both egg and sperm is reduced one half — on the one hand, 

 the ripe egg cell and three rudimentary egg cells (the polar 

 bodies) being formed; on the other, four equal "spermatids" 

 are produced, which develop into four mature spermatozoa. 

 The contrast in size which exists between the two mature re- 

 productive cells is enormous, the spermatozoon in some cases 

 containing less than tttoVoo (Wilson), and in extreme cases less 

 than To WxToo IT (Hertwig) of the volume of the egg cell. 



A . A i\^^ 



Fig. 151. — At left, diagram illustrating the development of the spermatozoon; at right, 

 diagram illustrating the development of the egg. (After Boveri.) 



A discussion of the method by which the reduction of the 

 chromosomes in the germ nuclei is brought about, may profit- 

 ably be -deferred until the essential features of fertihzation 

 have be^n examined. The phenomena of the fusion of egg and 

 sperm can best be studied in some such form as the sea urchin, 

 in which the egg is very small, and, in some species, quite 

 transparent. As fertilization takes place free in the sea water, 

 the germinal cells being cast out fK)m the parents, it is possible 

 to collect the eggs and sperm separately from mature in- 

 dividu^ils and bxing them together in small dishes of sea water, 

 r.nd at sucli times as may suit one's convenience. Then in the 

 r.ving egg much of the process may be followed under the 

 i.yL/oncopc, and properly prepared sections of the eggs, killed 



