98 GENERAL BIOLOGY 



particles which may be shaped like a horseshoe or like the capital letter 

 L, or merely appear as little straight or bent rods. These little broken 

 up particles of chromatin are called chromosomes. As these chromo- 

 somes are in all probability the most important physical particles in the 

 study of biology, one must get this subject of mitosis and chromosomes 

 clearly in mind or all that follows will necessarily be lost. 



Just before the cell goes from the resting stage into the skein or 

 spireme stage, the little centrosomes lying within the centrosphere break 

 into two parts, one part migrating around the nuclear wall until it lies 

 opposite the first half. 



Formerly it was thought that it was due to these polar bodies that 

 the chromatin breaks up into chromosomes, but as no centrosomes are 

 found in higher plants, although the chromatin acts just as it does in 

 animal cells, this explanation must be given up. Between these two 

 polar bodies in the animal cell there develops a series of very fine lines 

 which may be only a reflection of some kind, but which one very fre- 

 quently sees when the cell is undergoing mitosis. These fine lines are 

 called a spindle, readily recognized in the drawing. There are four 

 periods usually mentioned in cell division : 



The Prophases. This is the skein stage already mentioned. 



The Metaphase. Immediately after the chromosomes have appeared 

 as small broken particles of chromatin, they gather at the mid-line or 

 equatorial region of the spindles. Then the chromosomes split in two 

 .lengthwise and the cell is said to be in the metaphase stage. 



The Anaphases. Immediately after the chromosomes have divided 

 lengthwise, one-half of them move toward one polar body and the other 

 half toward the other. During the time the chromosomes have split 

 and the time they have united about the polar bodies, the cell wall has 

 indented until it meets the opposite indentation, thus forming two sepa- 

 rate daughter cells,* This stage is called the anaphase. 



The Telophases. This phase lasts from the anaphases until the 

 time the cells again resume the resting stage. 



It. will be noted that the metaphase is used in the singular, whereas 

 the other three have been used in the plural. This will be readily under- 

 stood when it is remembered that these terms are only convenient names 

 enabling us to discuss intelligently with others the whole subject of 

 mitosis, and, so that when a given thing or event is observed during 

 any particular time of the division of the cells, it can be written and 

 spoken about in an understandable way. 



The metaphase is only that particular moment when the chromo- 

 somes have gathered at the equatorial plane and are dividing. All the 

 other phases cover a much longer period, and, passing through various 

 , stages, are therefore used in the plural. 



*In plants a new cross cell-wall often originates by a thickening of the central spindle fibres. 



