28 Heredity and Eugenics 



among very closely related plants, or the number may be 

 constant throughout a great group. For example, in the 

 gymnosperms, so far as known, the number is almost con- 

 stantly 12. This fact has given rise to the suggestion that 

 the number of chromosomes, as well as their quality, may 

 be a factor in heredity, but too much stress must not be laid 

 upon it as yet. 



While the chromosomes are becoming separate, a spindle 

 of fibers is formed about the nucleus, and the chromosomes 

 become attached to the fibers, finally being arranged about 

 the equator of the spindle (Fig. 2, d). In this position, 

 each chromosome splits longitudinally (Fig. 2, e), and the 

 two halves, by the shortening of attached fibers, are drawn 

 toward the opposite poles of the spindle (Fig. 2, /), the 

 old chromosome thus being represented at each pole by a 

 half-chromosome. The half -chromosomes at each pole 

 enter into the organization of a new nucleus (Fig. 2, g), 

 wall material is deposited in the plane of the equator of the 

 spindle and, extending through the cytoplasm, cuts the old 

 cell into two new cells, each with its nucleus (Fig. 2, ti). 

 It is evident that each new nucleus has the same number 

 of chromosomes as the old one, and that each chromosome 

 of the new nuclei represents in material a chromosome of 

 the parent nucleus. 



This detailed and precise process in the behavior of 

 chromosomes, insuring the transmission from one cell to 

 its progeny cells of the identical material contained in 

 every chromosome, is a strong argument in favor of regard- 

 ing the chromosome as the carrier of hereditary qualities. 



The kind of reproduction with which the problems of 

 heredity are concerned chiefly, however, is that which 

 involves the fusion of sexual cells. There are three distinct 



