382 INTRODUCTION TO EVOLUTION 



(Fig. 17.2). As the primary spermatocyte prepares to divide, the chromo- 

 somes come together in pairs; e.g., the two chromatids representing the 

 maternal "long chromosome" pair with the two chromatids representing 

 the paternal "long chromosome." This pairing is called synapsis and during 

 it the chromatids are frequently twisted around each other instead of 

 lying smoothly side by side as shown in the diagram. At this time part of 

 one chromatid may be exchanged with part of another one. This exchange 

 is called crossing over and has important genetic consequences, as we 

 shall see below. 



Eventually each primary spermatocyte divides to form two secondary 

 spermatocytes. In this division each centromere remains intact, carrying 

 with it its two chromatids. Thus, in terms of our diagram, each secondary 

 spermatocyte receives the chromatids representing one "long chromosome" 

 and one "short chromosome." The chromatids representing the maternal 

 "long chromosome" (white) go to one secondary spermatocyte, those 

 representing the paternal "long chromosome" (shaded) go to the other. It 

 is important to note that what the "long" chromatids do in this respect does 

 not influence what the "short" chromatids do. In the diagram of the pri- 

 mary spermatocyte we have shown the maternal "long" chromatids on 

 the right, the paternal ones on the left, and the paternal "short" chromatids 

 on the right, the maternal ones on the left. This is a matter of chance. 

 About half the time this arrangement would be expected; about half the 

 time the maternal members of both "long" and "short" chromosomes 

 would line up on the same side, the paternal members of both on the other 

 side. 



Each secondary spermatocyte divides to form two spermatids. In this 

 division each centromere splits so that the chromatids separate, one 

 "long" chromatid and one "short" one going into each spermatid. Each 

 spermatid undergoes a metamorphosis, developing a swimming tail, and 

 becomes a mature sperm cell. We note that each sperm cell contains only 

 half as many chromosomes as did the spermatogonium and that one mem- 

 ber of each pair of chromosomes present in the spermatogonium is present 

 in the sperm cell. The number of singly occurring chromosomes in a 

 mature germ cell (e.g., sperm) is called the haploid number. In our 

 diagram the haploid number is 2. On the other hand, the number of 

 chromosomes occurring in pairs in primordial germ cells, and in body 

 cells, is called the diploid number. In our diagram the diploid number is 

 4. The process we have described is called meiosis; it results in the produc- 

 tion of haploid germ cells from diploid primordial ceUs. 



As noted above, we have pictured meiosis in a heterozygous black, 



