ANAPHASE 5 



disappears and the threads of chromatin become thicker. Sometimes the 

 original spindle remains ; but often it disappears and is replaced with a 

 new spindle formed within the nucleus. In many cells the chromatin 

 during the formation of the amphiaster assumes the form of a loose skein 

 between the two attraction-spheres, called the spirem. This is described 

 by some writers as the spirem stage. 



The threads of chromatin now separate into a number of shorter 

 threads called chromosomes, which arrange themselves in the equator of 

 the spindle, forming the equatorial plate. This completes the forma- 

 tion of what is known as the mitosic or karyokinetic figure. The 

 changes just described constitute the prophases of karyokinesis. All 

 embryologists agree that after the division of the chromatic threads the 

 number of resulting chromosomes for each species of plant or animal is 

 fixed and characteristic; and this is true after division of all its cells 

 (Wilson). The number in man is thought to be sixteen ; in sharks it is 

 thirty-six ; and in ascaris it is four or even two. Wilson gives a list of 

 forty-seven species in which the number has been noted by various 

 investigators. It is a curious fact, also, that the number is always divis- 

 ible by two. In fertilization, the number in the male and in the female 

 pronucleus is reduced one-half ; so that the cleavage-nucleus, which 

 results from the union of the two pronuclei, contains the same number 

 of chromosomes as did the original ovarian nucleus. 



Metaphase. The prophases just described are preparatory to the 

 processes resulting in cell-division, which begin with a splitting of each 

 chromosome lengthwise into two. This is called metakinesis, or the 

 metaphase. 



Anaphase. The arrangement of the chromosomes in the equatorial 

 plane of the spindle is different in different species of animals. In 

 many instances they take the form of the letter V and are arranged in 

 a cluster with their angles directed inward, so that the group forms 

 a sort of star. With this arrangement, the cell is in what is called 

 the monaster stage, and this stage often comes before the splitting of 

 the threads. Following this, the chromosomes pass one-half each, to the 

 attraction-spheres, around which they are grouped preparatory to the 

 formation of the daughter-nuclei. This is called by some the diaster 

 stage. In this stage, filaments pass between the two masses of chromo- 

 somes (interzonal fibres) and form the central spindle. The grouping 

 of the chromosomes around the two attraction-spheres marks the ana- 

 phase. It is probable that in most cells the fibres of the central spindle 

 are of new origin and are not derived from the spindle of the amphi- 

 aster. Lying in the equatorial plane of the central spindle is a group 

 of granules, constituting the mid-body. 



