6 TEXT-BOOK OF EMBRYOLOGY. 



chromosomes is that the number of segments into which the thread divides, 

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

 for each particular species. For example, in Ascaris megalocephala — a very- 

 convenient type for study on account of its simplicity — the number of chro- 

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

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



There are thus at this stage present in the cytoplasm, two distinct though 

 closely related structures — the amphiaster and the chromosomes. These 

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

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

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

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

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

 centrosomes (Fig. 3, C, D) : (1) the fibers of the central spindle connecting 

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

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

 centrosomes to the chromosomes. 



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

 formation marks the end of the prophase. 



Metaphase. — The essential feature of the metaphase is the longitudinal 

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

 containing an equal amount of the chromatin of the parent chromosome. In 

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

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

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

 tudinal splitting of the chromosomes is apparent when one considers that 

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

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

 nucleus of each daughter cell. 



Anaphase. — Actual division of the chromosomes having taken place, the 

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

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

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

 respective centrosome, around which the chromosomes finally become arranged 

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

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

 to undergo division at this stage, thus making four centrioles in the cell. 

 (Fig. 4, F, G.) 



Telophase (Fig. 4, H).— This is marked by division of the cytoplasm, 

 usually in the equatorial plane of the achromatic spindle, and the reconstruction 

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



