48 GENETICS 



vals. There are corresponding thickenings side by side in 

 the two threads (figure 18, A). 



12. The two chromosomal threads (maternal and pa- 

 ternal) forming a pair become intimately united. This 

 process is known as the conjugation, or synapsis, of the 

 chromosomes. (During conjugation the two chromosomes 

 may exchange parts, by "crossing-over," as will be de- 

 scribed later.) 



This intimate union of the maternal and paternal chro- 

 mosomes appears to be the ultimate act in the union of the 

 sexes. 



13. Next occur certain processes, the general result of 

 which is simple, although the processes are complex. The 

 final result Is that which would be produced if the imma- 

 ture germ cell now divided but once into two final germ 

 cells, and in so doing, the two chromosomes of each pair 

 separated, one going to one of the two germ cells, one to the 

 other (figure 4). Thus each mature germ cell contains fi- 

 nally one chromosome (the maternal one or the paternal 

 one) from each pair. The total number of chromosomes 

 in the germ cell Is now one-half the number originally pres- 

 ent in the immature germ cell; it is n in place of 2«. This 

 is the process known as reduction of the number of chromo- 

 somes. 



The actual processes are more complex, and are some- 

 what confusing. In following them, the final result just set 

 forth should be kept in mind. They are as follows (fig- 

 ure 18) : 



14. During conjugation the two long chromosomal 

 threads shorten and thicken (figure 18, A, B). 



15. And while this Is occurring, each of the two splits 

 into two, so that there are now four partly united threads, 

 two maternal, two paternal, in origin (see figure 18, C). 



16. The shortening and thickening continues till the 

 four threads have gathered into four closely united lobes 



