THE CELLULAR BASIS 117 



egg and sperm nuclei run together into a 

 smooth thick thread, the spireme, which is 

 coiled within the nucleus. At this stage it is 

 sometimes possible to see that the spireme is 

 composed of a series of granules, like beads on 

 a string; these granules are the chromomeres 

 (Fig. 4s L). The spireme then breaks up into a 

 number of pieces in the form of short threads 

 or rods (Fig. 24 C and D) ; these are the 

 chromosomes. The number of these chromo- 

 somes is constant for every species and race, 

 though the number may vary in different 

 species. In the thread worm, Ascaris mega- 

 locephala, there are usually two chromosomes 

 in the egg nucleus and two in the sperm nu- 

 cleus (Fig. 24 D) . In the gastropod, Crepid- 

 ula, there are about thirty chromosomes in 

 each germ nucleus and sixty in the two. 



Then the spindle and asters grow larger and 

 the nuclear membrane grows thinner and 

 finally disappears altogether, leaving the chro- 

 mosomes in the equator of the spindle (Figs. 

 5A, 23 F, 24 E and F) . Each of the chromo- 

 somes then splits lengthwise into two equal 

 parts, and in the splitting of the chromosomes 



