6 TEXT-BOOK OF EMBRYOLOGY. 



chromosomes is that the number of segments into which the thread divi 

 while differing for different species of plants and animals, is fixed and defini 

 for each particular species. For example, in Ascaris megalocephala a v 

 convenient type for study on account of its simplicity the number of c 

 mosomes is 4, in the mouse 24. In man the number is not known A 

 certainty; by some it is estimated at 16, by others at 24. 



There are thus at this stage present in the cytoplasm, two distinct tho.ig 

 closely related structures the amphiaster and the chromosomes. T] e 

 together constitute the mitotic figure. As the chromosomes form they becon 

 arranged in the equator of the central spindle, along what is known as t 

 equatorial plane (Fig. 3, D). When, as is frequently the case, the chromosome! 

 are U-shaped, the closed ends of the loops lie toward the center, the open e 

 radiating. Three sets of fibers can now be distinguished in connection with t j 

 centrosomes (Fig. 3, C, D) : (i) the fibers of the central spindle connec 

 the two centrosomes; (2) the polar rays which radiate from the centros< 

 toward the periphery of the cell; (3) the mantle fibers which pass from 

 centrosomes to the chromosomes. 



The mitotic figure is at this stage known as the monaster, and its complei 

 formation marks the end of the prophase. 



METAPHASE. The essential feature of the metaphase is the longitud 

 splitting of each chromosome into exactly similar halves (Fig. 4, E), each ] 

 containing an equal amount of the chromatin of the parent chromosome, 

 the case of U- or V-shaped chromosomes, the splitting begins at the crow 

 and extends to the open ends. The latter often remain united for a tim 

 giving the appearance of rings or loops. The significance of this equal loi 

 tudinal splitting of the chromosomes is apparent when one considers t 

 through this means an exactly equal part of each chromosome and thus exa< 

 equivalent parts of the chromatin of the parent nucleus are distributed to 

 nucleus of each daughter cell. 



ANAPHASE. Actual division of the chromosomes having taken place, 

 next step is their separation to form the daughter nuclei. In separating, 

 daughter chromosomes pass along the fibers of the central spindle (Fig. 4, 

 apparently under the guidance of the mantle fibers, each group toward i 

 respective centrosome, around which the chromosomes finally become arran| 

 (Fig. 4, G), thus forming two daughter stars. The mitotic figure is now kno- 

 as the diaster. In actively dividing cells it is common for the centrosome 

 undergo division at this stage, thus making four centrosomes in the c 

 (Fig. 4, F, G.) 



TELOPHASE (Fig. 4, H). This is marked by division of the cytoplae 

 usually in the equatorial plane of the achromatic spindle, and the reconstruct: 

 of the two daughter nuclei. Each new cell now contains a nucleus a centroso 



