240 ANIMAL BIOLOGY 



some detail the complicated internal process involved in all typical 

 cell divisions, known as mitosis, which was dismissed when con- 

 sidering the origin of cells until the reader would be in a position 

 to appreciate to the full its significance. (Fig. 10.) 



Reduced to its simplest terms, a typical resting cell, that is one 

 which is not dividing, consists of a mass of cytoplasm surrounding 

 a nucleus; the latter with its chromatin distributed so that it 

 presents a net-like appearance. In addition to the nucleus, it will 

 be recalled that there is present another important cell organ, the 

 centrosome, which appears like a tiny body enclosing a granule 

 and is situated in the cytoplasm near the nucleus. For practical 

 purposes we may consider the cytoplasm as the arena in which 

 mitosis takes place, the centrosome as the dynamic agent, and the 

 nucleus, or more specifically its chromatin, as the essential element 

 which the complicated process is to distribute with exactness to 

 the daughter cells that are about to be formed. With this in mind 

 we may proceed to an outline of the chief stages of mitosis, though 

 perhaps it should be emphasized that variations in the details are 

 as numerous as the different types of cells, and that any general 

 account can do no more than present the fundamental plan of 

 operations. 



Broadly speaking, mitosis can be divided into four chief stages : 

 prophase, metaphase, anaphase, and telophase, during each of 

 which characteristic changes take place in the nucleus, cytoplasm, 

 and centrosome. (Fig. 164.) 



At the beginning of the prophase, or earlier, the centrosome 

 divides to form two, each of which becomes surrounded by what 

 appears to be a halo (aster) of radiating fibers, the nature of 

 which is unknown, that are the visible expression of physico-chemi- 

 cal forces. The centrosomes and asters now T proceed to move apart, 

 take up positions at opposite sides of the nucleus, and the astral 

 fibers between lengthen until they form a central spindle. While 

 these changes are going on, the nucleus is not inactive. The nuclear 

 membrane gradually disappears and the chromatin granules, orig- 

 inally presenting a net-like appearance, now become visibly re- 

 solved into a number of split threads of chromatin, termed chro- 

 mosomes, which by chromatin concentration gradually become 

 shorter and thicker and so distinctly individual. The number of 

 chromosomes varies greatly in different species, but is typically 

 an even number and the same for all the cells of a given species. 



