3o8 



ELEMENTS OF BIOLOGY 



them. While the asters are thus migrating the chromatin granules 

 within the nucleus become arranged into a distinct thread, known 

 as a SPIREME because o£ its coiled arrangement, and the membrane 

 around the nucleus begins to disappear, being transformed by some 

 unknown process of metabolism. The spireme presently is resolved 

 into a number of rods or other shaped units of chromatin known as 

 CHROMOSOMES. As the nuclear membrane disappears these chromo- 

 somes that have resulted from the breaking up of the spireme come 

 to position halfway between the asters. Meanwhile fibres have 

 formed between the asters, which gives the whole complex of asters, 



fibres, and chromosomes the ap- 

 pearance of a spindle with a scat- 

 tered group of dense bodies dis- 

 tributed about on a plane (the 

 equatorial plane) at right angles 

 with the long axis of the spindle 

 and halfway between the two poles. 

 This is known as the mitotic spin- 

 dle; its appearance marks the close 



of the PROPHASE. 



The Metaphase (Fig. 198). 

 Chromosomes. The second stage is the metaphase. Before 

 describing it, however, it is necessary to give some attention 

 to the chromosomes that have become so distinct during the 

 prophase. Each species has a fixed number of these bodies in 

 each cell during mitosis, except as will be seen later, in certain 

 stages of the development of the gametes. Some chromosomes 

 are rods, others are the shape of open hooks, others are spheri- 

 cal; all have characteristic shapes and it is possible to identify 

 the chromosomes that descend from specific chromosomes through 

 generation after generation of cell divisions. Thus, although the 

 chromatin is scattered in a non-dividing cell, chromosomes always 

 appear during the prophase in their characteristic form and number 



Fig. 198. — The metaphase. 



