36 REPRODUCTION AND DEVELOPMENT. 



The chromatin develops into an apparently continuous and 

 single long filament in a tangled skein, or dense convolution, or 

 " spirem." This is the stage of the continuous convolution. At 

 first the chromatin-filament is fine, and is arranged in close, 

 crowded turns ; this formation is called the close skein. Later, 

 the filament becomes coarser, and the loops and windings of 

 the convolution more open, forming the open skein. The 

 nuclear membrane and nucleoli become gradually transformed 

 into the chromatin-filament and absorbed into the skein, and 

 thus disappear, so that no wall now separates the nucleus from 

 the cell-body. 



The chromatin-filament, which heretofore has apparently 

 been one continuous thread, then breaks up into segments, 

 forming the segmented convolution. Each of these segments is 

 called a chromosome. The chromosomes form V-shaped or 

 U-shaped loops, and tend to arrange themselves with the 



FIG. 9. 



Cell (ovum of ascaris megalocephalus) undergoing karyokinetic division (Kostanecki 

 and Siedlecki). a, centrosome ; b, chromosomes. 



angles toward a common centre and the arms radiating out- 

 ward. The number of chromosomes in different species is 

 usually from four to thirty-six in each cell ; one crustacean, 

 artemia, has 168. Human cells contain about sixteen chromo- 

 somes each. The number of chromosomes is uniform in all 

 the cells of the same species. In the meantime, during the 



