THE CELLULAR BASIS 111 



thick thread, the spireme, which is coiled within 

 the nucleus. At this stage it is sometimes pos- 

 sible to see that the spireme is composed of a 

 series of granules, like beads on a string ; these 

 granules are the chromomeres. 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 chromosomes is constant for every 

 species and race, though the number may vary 

 in different species. In the thread worm, 

 Ascaris megalocephala, there are usually two 

 chromosomes in the egg nucleus and two in the 

 sperm nucleus (Fig. 24 D). In the gastro- 

 pod, Crepidula, there are about thirty chromo- 

 somes 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. 

 23 F, 24 E and F, 25 7) . Each of the chromo- 

 somes then splits lengthwise into two equal 

 parts, and in the splitting of the chromosomes 



