3l6 GENERAL PRINCIPLES 



opposite directions, each one approaching one of the centro- 

 somes. In this way two groups of chromosomes of equal 

 numbers are formed at opposite ends of the spindle. At about 

 this time a groove appears in the surface of the cytoplasm in 

 the equatorial plane of the spindle. This groove cuts deeper 

 into the cell until it is divided into two equal masses. Thus the 

 nucleus and cytoplasm are divided. 



685. The process of division is concluded by the formation of 

 a nuclear membrane around each group of chromosomes and the 

 rearrangement of the chromatin. The chromosomes lose their 

 individuality again in a tangle of chromatin and the nucleoli 

 reappear. The spindle fibres and attraction sphere disappear 

 and the centrosomes may also be lost among the other granules 

 of the cytoplasm. 



686. This process of cell division is called mitosis or karyo- 

 kinesis. It is the normal method of cell division, but under 

 certain conditions a simpler process occurs. This has been 

 described elsewhere. With slight modification the description 

 just given of the mitotic method will apply generally to both 

 animals and plants. Several additional points may be 

 mentioned. 



687. Number of Chormosomes. — The number of chromo- 

 somes is constant for any given species. For different species 

 the numbers observed vary from two in the Nematode, Ascaris 

 megalocephala, to 168 in Artemia, a genus of Crustacea. The 

 most common numbers recorded are 12, 16 and 24. 



688. Nucleoli. — The fate of the nucleoK during mitosis is 

 in question. There is some reason for believing that in some 

 cases at least they take part in the formation of the chromo- 

 somes. More often they seem to disintegrate, and then new 

 ones are formed on the organization of the new nucleus. 



689. Centrosomes. — The centrosomes are apparently per- 

 manent cell structures which propagate themselves by divi- 

 sion. At the close of cell division the centrosome of each 



