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THE POPULAR SCIENCE MONTHLY 



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. 

 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 



Fig. 24. Fertilization of the Egg op the Nematode Worm Ascarts; <£N, egg 

 nucleus ; gN, sperm nucleus ; Arch, archiplasm ; (7, centrosome ; A, B, approach of 

 germ nuclei ; C, D, formation of two chromosomes in each germ nucleus E, F, stages 

 in the division of the chromosomes which are split in E and are separating in F ; 

 only three chromosomes are shown in F. (From Wilson after Boveri.) 



number of these chromosomes is constant for every species and race, 

 though the number may vary in different species. In the thread worm, 

 Ascaris, there are usual] y two chromosomes in the egg nucleus and two 

 in the sperm nucleus (Fig. 24 D). In the gastropod, Crepidula, 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 



