104 BIOLOGY: GENERAL AND MEDICAL 



chromatin. If a nucleolus be present, it appears as a mi- 

 nute distinct dot. As the chromatin increases in quantity 

 prior to division, it comes to fill the greater part of the 

 nucleus and the nucleolus disappears. Soon the chroma- 

 tin increases in filamentousness until a single long, some- 

 what spirally coiled thread the spirem is formed. When 

 examined under most appropriate conditions this thread 

 has a fuzzy appearance, which is supposed to depend 

 upon an incomplete differentiation of the chromatin from 

 the linin, which clings to it. Soon, however, the linin 

 separates completely, after which it is observed that the 

 chromatin no longer forms a single thread, but has broken 

 into a number of segments of uniform length, which are 

 the chromosomes. These bodies, first described in 1888 

 by Waldeyer, are of great interest from many points of 

 view, and are believed by Weismann and others to endow 

 the offspring of the cell with its chief hereditary impulses. 

 The number of chromosomes is usually the same in all of 

 the cells of the same organism, though it varies in different 

 kinds of organisms, and in the two sexes of any kind of 

 organism. When, as hi the cells of vertebrates, and es- 

 pecially mammals, the number of chromosomes is great, 

 there is much difficulty in counting them. Thus, the 

 somatic cells of human beings, oxen, and guinea-pigs are 

 usually said to have sixteen chromosomes; those of the 

 mouse, salamander, and trout, twenty-four. Mont- 

 gomery, who painstakingly studied the spermatocytes of 

 man, thought that they contained twenty, twenty-two, 

 or twenty-four chromosomes; but Winiwarter, whose 

 work is more recent, believes that there are forty-seven 

 in the human male and forty-eight >in the human female. 

 In complex organisms there is also a difference in shape 

 between the chromosomes of the somatic and germinal 

 cells, by which the latter are, at one time, made to appear 

 twice the number of the former, and at another time are 

 reduced to one-half the number of the former. 

 In cells in which no centrosome is usually visible, that 



