CHROMOSOMES 251 
share of the chromatin which was present in the 
parent nucleus. 
A great deal of evidence has recently accumulated 
to show that chromosomes are very definite and 
important organs. In the first place, the number of 
chromosomes which make their appearance at each cell 
division is the same in all the cells of any given | 
creature, and this numerical constancy further extends 
to the cells of all the members of a particular species, 
though in members of allied species the number of 
chromosomes may be different. In widely separated 
species the number of chromosomes varies consider- 
ably ; thus from 2 to 200 have been counted in the 
case of various different members of the animal and 
vegetable kingdoms. One of the commonest numbers 
found fs twelve, and this number occurs in a con- 
siderable variety of different animals and plants. 
Next it has been shown that the chromosomes 
which arise at the beginning of a nuclear division are 
identical with those daughter chromosomes of the 
preceding division which originally entered into the 
nucleus now about to divide. An example of the 
kind of evidence upon which this conclusion is based 
may next be given. 
Figs. 27, 28, and 29 show the three possible arrange- 
ments of the four chromosomes which are found in the 
cells of the worm-like animal Ascaris, as seen from the 
direction of the pole of the spindle in the dividing 
nucleus. Of these arrangements, that: shown in 
Fig. 29 is much the least common. Now in this par- 
ticular case the chromosomes, when they first make 
