116 



EVOLUTION AND GENETICS 



The explanation of crossing-over rests on the 

 assumption that members of the same pair of chro- 

 mosomes may at times interchange. If the chromo- 

 somes are the bearers of the genes there can be no 

 doubt from the genetic evidence that such an inter- 

 change takes place. When we turn to the known 

 behavior of the chromosomes in the ripening of the 

 germ cells we find certain stages where such a pro- 

 cess may seem possible. 



At the ripening period of the germ cell the mem- 

 bers of each pair of chromosomes come together. In 

 several forms they have been described as meeting 

 at one end and then progressively coming to lie side 

 by side as shown in figure 51. At the comj^letion of 



Fig. 51. — Conjugation of the chromosomes in Batracoceps. 

 (After Janssens.) 



the process they appear to have united along their 

 length. It is always a maternal and a paternal chro- 

 mosome that meet in this way and always two of the 



