222 THE CONTINUITY OF THE RACE 



secondary spermatocyte now undergoes the second meiotic division, again 

 followed by an even division of the cytoplasm, to result in the formation 

 of four spermatids. These develop without further division into functional 

 spermatozoa. The nucleus becomes even more condensed, and the 

 cytoplasm is organized into the whiplike propulsive tail of the mature 

 spermatozoon. 



The matured spermatozoon. At the completion of spermatogenesis 

 the spermatozoon is ready to leave the testis and is capable of fertilizing 

 an egg. It is very small, usually less than one-thousandth the bulk of the 

 ovum, is capable of locomotion, and has the property of being attracted 

 toward a substance or substances that diffuse from the unfertile eggs of 

 its own species. 



Fertilization of the egg. In the vast majority of animals sexual 

 reproduction is bisexual; the matured egg is incapable of development 

 until it has been fertilized by a spermatozoon. When spermatozoa are set 

 free sufficiently close to such an unfertilized egg, they are chemically 

 attracted by it and swim in its direction. Only one spermatozoon can 

 fertilize an egg. The first one to reach it penetrates the egg membrane, 

 and the head (nuclear portion) of the spermatozoon moves through the 

 cytoplasm to meet the egg nucleus. Once within the egg membrane the 

 sperm nucleus absorbs fluid and swells to the size of the egg nucleus, so 

 that it soon becomes difficult to distinguish between the two. Soon after 

 the two nuclei come into contact, they fuse to form a single diploid 

 nucleus, which contains one haploid set of chromosomes from the egg and 

 another haploid set from the sperm nucleus. The diploid number of 

 chromosomes that characterized the unmatured germ cells (and soma 

 cells) is thus restored. Fertilization is now complete, and the fertilized 

 egg or zygote (Greek, zygotos, "yoked together") is ready to begin em- 

 bryonic development. 



BISEXUAL AND UNISEXUAL REPRODUCTION 



We have already defined sexual reproduction as reproduction by germ 

 cells and have seen that it normally involves the union of sperm and egg 

 cells produced as a result of meiosis and maturation. Up to this point we 

 have been concerned with the cells involved; now let us consider the 

 individuals in which these cells are produced, and some of the variations 

 in sexual reproduction. 



Dioecious versus hermaphroditic conditions. We ordinarily think of 

 bisexual reproduction as being correlated with the existence of two kinds 

 of individuals — males that have testes and produce spermatozoa, and 

 females that have ovaries and produce eggs. The sperms and eggs live, 

 so to speak, in different "houses," and we speak of organisms with sepa- 

 rate male and female individuals as being dioecious (Greek, di, "two," 



