HISTOLOGY 



slender observation in the chick. His observations, however, were 

 made on tissues that divided by mitosis, and he probably mistook some 

 later phases of this process for a process superficially comparable to 

 amitosis. 



Amitosis usually begins in a cell by the elongation of the nucleolus at 

 right angles to the future plane of division of the cell. A constriction 

 of the middle of this organ then proceeds, not as though some power was 

 cutting it in two as a band or string would do, but more as though the 

 two ends were being pulled apart and the middle was thinning out to 

 the breaking point. The two daughter nucleoli then move apart to posi- 

 tions in the approximate centers of the two future daughter cells, and the 

 nucleus is ready to divide. Frequently the daughter nucleoli begin to 

 elongate as though for another division before the mother nucleus has 

 even begun to divide. 



In other cases the nucleus acquires the two daughter nucleoli not by 

 the splitting of the old one but by the growth of a new one in situ at the 

 opposite side of the nucleus from the original nucleolus. This method 

 often results in a splitting of the nucleus before the new nucleolus is as 

 large as the other, and one of the new cells is then smaller than its sister- 

 cell. 



The division of the nucleus, subsequent to that of the nucleolus, 

 may be done in one of three ways, the first and second of which are much 

 alike. It may divide by a constriction of its body, as in the case of the 

 nucleolus : this, however, is rare. Oftener it forms a plate at the plane 



of division, and this becoming 

 double, the two plates separate, 

 showing flat and parallel surfaces 

 where the separation took place. 

 The third method of nuclear divi- 

 sion, recently suggested by Child, 

 1907, from numerous observations 

 on many forms, is performed by 

 the formation of two new nuclear 

 membranes inside the old one and 

 around the daughter nuclei. This 

 is strongly suggested by his figures, 

 one of which is copied in our Fig- 

 ure 44. The dissolution or absorp- 

 tion of the old membrane would 

 then leave the two daughter nuclei free in the cytoplasm and separate 

 from one another. 



The cell body is last to divide in amitosis, and in many, perhaps the 

 majority of cases, it does not do so at all. Or it may form a division 



FIG. 44. Amitotic division in the tissues of a 

 flat worm, Planaria maculata. (After CHILD.) 



