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 iWouir (Wilson), and in extreme cases less 

 than ToWwoo- (Hertwig) of the volume of the egg cell. 



v 



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 fertilization 

 have been 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 from the parents, it is possible 

 to collect the eggs and sperm separately from mature in- 

 dividuals and bring them together in small dishes of sea water, 

 and at such times as may suit one's convenience. Then in the 

 living egg much of the process may be followed under the 

 microscope, and properly prepared sections of the eggs, killed 



