98 General Biology 



is, after the male sperm has united with the female egg. The chromatin 

 (stainable nuclear network) begins a process by which the stained part 

 separates from the rest of the network, taking upon itself the shape of 

 a single thread or skein. A little later, this skein of chromatin breaks 

 up into small particles of various shapes. Some of the more common 

 shapes are those bent like a horseshoe or like the capital letter L, and 

 those that appear as little straight or bent rods. Such portions of 

 chromatin are called chromosomes. As these chromosomes 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 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 centrosomes 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 the two 

 centrosome parts in the animal cell there develops a series of very fine 

 lines, which may be only a reflection of some kind, but which are very 

 frequently seen when the cell is undergoing mitosis. These fine lines 

 are called a spindle, readily recognized in the drawing. Four periods in 

 cell division are usually mentioned : 



The Prophases. This is the skein stage already referred to. 

 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 centrosomes, the cell wall has 

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

 rate daughter cells. 1 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- 



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



