THE CHROMOSOMES 129 



is some dissent, that the first longitudinal split that 

 appears in the thick thread (pachytene stage) lies 

 between the two chromosomes that had previously 

 come together, such a separation of the members of a 

 pair of chromosomes being known as a reductional 

 split. The second lengthwise split is supposed to 

 separate like halves of the same chromosomes. It is 

 called an equational split. 



These two sphts are in preparation for the two 

 maturation divisions that usually take place in rapid 

 succession, without an intervening resting stage. 

 It is customary therefore to look upon the second 

 lengthwise split as a precocious split in the chromo- 

 somes preparatory to the second division. If the 

 reduction in the number of the chromosomes to half 

 of the original number were the sole object of the 

 reduction divisions, one division would suffice to 

 separate the two chromosomes of a pair that had 

 united and it is not apparent why there should be a 

 second division at all. 



The two maturation divisions with tetrad forma- 

 tion are typically illustrated in the changes that take 

 place in the spermatogenesis and oogenesis of 

 Ascaris, the thread worm of the horse, as worked out 

 by van Beneden, Brauer, 0. Hertwig and others. 

 In one variety four chromosomes occur which become 

 reduced to tw^o; hence there are only two tetrads 

 present (Fig. 45, a). At the first division two halves 

 of each thread move to one pole and two to the 

 other as in h and c. At the second division the 

 separation of the two remaining threads takes place, 



