CELL DIVISION 



59 



approaches completion. During these changes the new nucleus may 

 rotate considerably in the cytosome, as it is shown to have done in the 

 illustration. In the figure (J) the two cells are shown in different stages 

 of the reconstruction process. This is done merely to illustrate the 

 steps involved, for as a rule they transform at about the same speed and 

 are at all times in about the same stage. 





m 



A B C 



Fig. 36. — Chromosomes of various shapes and sizes shown just before they are arranged 

 across the middle of the spindle. A, oogonium of the beetle Dytiscus (from Dehaisieux 

 in La Cellule); B, spermatogonium of arrow worm Sagitta (from Bar das in La Cellule); 

 C, egg of hellbender (from B. G. Smith in Journal of Morphology and Physiology) . 



The principal other features of the reconstruction are the loss of the 

 remaining spindle fibers in the cytosome and the formation of a nucleolus 

 if there was one prior to division (J) . When these steps have been taken, 

 two new cells of smaller size, essentially identical with one another, have 

 been produced from one older cell. 



Variations Relating to Chromosomes. — While the foregoing account 

 represents a fairly typical mitosis, there are many variations in the 

 process. The number of chromosomes differs greatly in different species. 



Fig. 37. Fig. 38. 



Fig. 37. — Splitting of the chromosomes before the equatorial plate stage; peritoneum 

 of the salamander Ambystoma. (From Parmenter in Journal of Morphology.) 



Fig. 38. — Reconstruction of nuclei through imbibition of liquid by the chromosomes 

 to form vesicles. A and B, early and late stages of vesiculation in the egg of the sea urchin, 

 in which the vesicles fuse. (From Danchakoff in Journal of Morphology.) 



In the parasitic worm Ascaris megalocephala each cell has 4 chromosomes; 

 in the vinegar fly Drosophila melanogaster the number is 8; and man 

 has 48 chromosomes. Most of the numbers from 4 to 60 are found in 

 one or more species, and there are some numbers above and below these 

 limits. The number differs in the two sexes in some animals, being 

 usually more numerous in the female when there is such a difference. 



