128 
Alice M. Borin^ 
fifth divisiou of tlie cells of the germ track, and show the same sized 
small chromosome. 
On the other liand, there is Variation in size. Even in a worm 
vphere one finds most of the small chromosomes comparable, there 
are always some that are too short or too long to he the same mor- 
phological unit, even allowing a large margin for difference in degree 
of contraction. Figures 15, 16, 17 and 18 are from the same worm. 
Taking all the worms where it is present, into consideration , this 
Variation in size is more striking than in the eggs from any one 
worm. When we compare figure 19 with figure 1, it seems very 
improbable that this is one and the same definitive element, and the 
only explanation seems to he that it is in one or the other case or 
in both cases due to a cliance fragmentation. 
Number. 
If this small chromosome comes from a fragmentation of one of 
the large chromosomes , the question arises why is there usually only 
one present? In only seven eggs have two been found, and never 
three or four. It wonld seem that, if a chromosome can break ofl‘ 
at one end, it could just as easily break at both ends and that if 
one chromosome can break to pieces, so could all four. Figure 18 
shows two small chromosomes of the same size, while in figure 17, 
they are of different sizes. On the other band, if it is a chromo- 
some unit with definite hereditary material such as the heterochromo- 
somes of insects, why sliould there sometimes he one and some- 
times two? 
Fragmentation. 
Beside the above negative evidence that points to this small 
chromosome as a case of fragmentation, there is some positive evi- 
dence. Figures 15 and 16 show one chromosome strikingly short 
and the small chromosome in such a position that it could easily 
have just broken off. Also in the same figures, it is a striking fact 
that the shortest of the long chromosomes has one end thinner than 
the other, more like the middle of a chromosome than an end, and 
the small chromosome adjacent is thick like an end. These are by 
no means isolated cases. Some such cases were found in all the 
worms with small chromosomes, and a great many in some worms. 
Figure 2 is from another worm, and shows evident fragmentation. 
