162 THE CHROMOSOMES 



to correspond to the former line of union of the 

 conjugating threads (reductional spht); the other 

 spht is then said to correspond to a division within 

 each thread (equational spht). It is obvious, how- 

 ever, if crossing over has previously taken place 

 between the threads, or between strands only, that 

 these distinctions apply rather to segments of the 

 chromosomes than to the chromosomes as wholes. 



These two splits 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 two; 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 b and c. At the second division the 

 separation of the two remaining threads takes place, 



