CELL DIVISION AND GAMETOGENESIS 221 



centrioles are usually not visible in sections of adult tissues. 

 During rapid cell divisions they continue from one cell generation 

 to the next, but there is evidence that they may be formed 

 de novo from the cytoplasm. 



Division Stages. — The following description is in the nature 

 of a generalized account of the mitotic process as it occurs in 

 somatic cells. There are many differences in detail in mitosis in 

 different animals, which can be only touched upon here. The 

 preparatory steps leading to cell division make up the prophase 

 of the process, which is initiated apparently by the separation 

 of the centrioles and the formation of a mitotic spindle between 

 them (Fig. 137). This is often accompanied by the formation of 

 astral radiations extending in all directions from the centrosomes. 

 The chromatin of the nucleus meanwhile condenses into a number 

 of thin threads, corresponding to the number of chromosomes 

 characteristic of the species. In some forms the chromatin 

 threads seem at first to be connected in a continuous thread, 

 known as a spireme, which later breaks up into chromosomes ; in 

 others the chromatin threads appear separate from the beginning 

 of their formation. In either case a definite number of chromo- 

 somes forms from the chromatin of the resting nucleus, regardless 

 of whether a continuous spireme is formed or not. Chromosomes 

 are frequently bent in V shapes, as shown in Fig. 137. The 

 details of mitosis can be followed more readily in the latter type 

 of chromosome, which for this reason has been used in the 

 illustration. 



As the chromosomes assume form, the centrioles move farther 

 apart with an accompanying enlargement of the spindle which 

 gradually shifts to the center of the cell. The nuclear membrane 

 then gradually dissolves, releasing the chromosomes, which 

 quickly assume places in the equatorial plane of the spindle. In 

 some forms, the chromosomes appear as undivided rods, or each 

 may show a longitudinal split that foreshadows the ensuing 

 division, and which may appear in the chromosome while it is 

 still within the nucleus. In the former case the undivided 

 chromosomes become split shortly after reaching the spindle. 

 The nucleolus melts away with the dissolution of the nuclear 

 membrane and takes no part in the division process. When the 

 chromosomes have become arranged in the equatorial plane of 

 the spindle, the metaphase stage of mitosis has been reached 



