56 



PRINCIPLES OF ANIMAL BIOLOGY 



is so evident in these glands undoubtedly exists elsewhere. One indi- 

 cation of this is found in animals which have different shapes and sizes 

 of chromosomes. At every division there is the same number of chromo- 

 somes of a given shape and size, which could hardly be true unless the 

 chromosomes maintained their identity in the intervening interphase. 

 Also, chromosomes may be broken up by X rays, and reconstituted in 

 new sizes and shapes, and these new chromosome forms appear again 

 after cell division and in later generations. Obviously chromosomes 



Fig. 35. — ^Mitotic cell division. A, cell not in division; B, centrioles move apart; C, 

 distinct chromosomes formed; D, nuclear membrane dissolved, spindle completed; E, F, 

 equatorial plate, side and end view, with chromosomes duplicated; G, H, chromosomes 

 move apart; /, /, division of cytosome and construction of new nuclei. (A, interphase; 

 B-D, prophase; E, F, metaphase; G-I, anaphase; J, telophase.) 



maintain their individuality in the interphase, even though it cannot 

 be observed. 



Prophase. — Mitosis is nearly enough alike in most cells to make 

 possible a general account of the process. Starting with a cell in inter- 

 phase, in which the centriole is already divided into two parts, one of 

 the early signs of division is the condensation of the chromatin into 

 distinct threads tangled about in the nucleus (Fig. 35/?). In whatever 

 way the chromosomes (page 26) are spread out through the nucleus, 

 they now contract into smaller compass, usually in the form of slender 

 strings or ribbons. The parts of the centriole' separate and move toward 

 opposite sides of the nucleus. Sometimes between them a few threadlike 



