162 THE CHROMOSOMES 
to correspond to the former line of union of the 
conjugating threads (reductional split); the other 
split is then said to correspond to a division within 
each thread (equational split). 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 odégenesis of 
Ascaris, the thread worm of the horse, as worked out 
by van Beneden, Brauer, O. 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 6 and c. At the second division the 
separation of the two remaining threads takes place, 
