56 THE CELL 



Late in the prophase the nucleolus and nuclear membrane disappear. 

 The cytoplasm and karyoplasm then become continuous and both 

 spireme and amphiaster lie free in the general cell protoplasm (Fig. 



9, C). 



3. The spireme next breaks up into a number of segments — 

 chromosomes (Fig. 9, C). These are usually rod-shaped at first, 

 later they may become U's or V's or may even become spheroidal. 

 The chromosomes now arrange themselves regularly around the equa- 

 tor of the nucleus, their closed ends being directed centrally. 



The details of the transformation of the reticulum into chromo- 

 somes vary. In some cases a single spireme-thread is formed. In 

 others the spireme-thread first splits longitudinally into two threads 

 before segmenting into chromosomes. Again the spireme-thread 

 may show segmentation into chromosomes from the beginning. 

 In still other cases the chromosomes apparently form directly from 

 the reticulum without the intervention of the spireme stage. It is 

 most important to note that while the number of chromosomes 

 varies for difi'erent species of plants and animals, it is fixed and 

 characteristic for a given species. Thus in Ascaris megalocephala 

 (much used for study on account of its small number of chromosomes) 

 the number is 4, in the mouse 24, in man, the most authoritative 

 estimate is 24.^ This means that whenever mitosis occurs in Ascaris, 

 the spireme-thread invariably segments into 4 chromosomes. Chro- 

 mosomes and amphiaster now constitute the jnitotic figure which at 

 this stage is known as the monaster, its formation marking the end 

 of the prophase. 



(b) Metaphase (Fig. 10, E). This marks the beginning of actual 

 division of the nucleus. Each chromosome spHts longitudinally 

 (longitudinal cleavage) into two daughter chromosomes, each contain- 

 ing exactly one-half the chromatin of the parent chromosome. U- 

 and V-shaped chromosomes always begin to spHt at the apex, from 

 which point the separation extends to the open ends. 



(c) Anaphase (Fig. 10, F, G). — An equal number of daughter 

 chromosomes now travels along the fibrils of the achromatic spindle — 

 apparently under the influence of the mantle fibres — toward each 



1 Gayer (Biol. Bui. Marine Biol. Lab. Wood's Hole, Mass., Vol. XIX) describes 

 twenty-two chromosomes in human spermatogonia, of which two are "accessory". 

 Apparently half the resulting spermatids contain ten chromosomes, the other half 

 twelve, two of which are accessory. In Syromastes Wilson found an identical condi- 

 tion and it was later determined that the somatic number of chromosomes for Syro- 

 mastes was twenty-two for the male and twenty-four for the female. Guyer concludes 

 that the probable somatic number for the human male is twenty-two, for the femak- 

 twenty-four. 



