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 Is 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 mak$ 



