i6 



NORMAL HISTOLOGY. 



FIG. 7. 

 P 



the brilliant investigations of Flemming, Strasburger, v. Beneden, 

 Schleicher, Rabl, and others, who, by the employment of improved 

 optical appliances and methods of investigation, have added much 

 to the accurate knowledge of the life-history of the cell. 



When the cell undergoes a complete and typical mitotic division, 

 the following changes occur: 



(i) The nucleus becomes larger, and, at the same time, the chro- 

 matin greatly increases, the fibrils becoming con- 

 torted to form a dense convolution, whose twisted 

 threads run generally transverse to the long axis 

 of the nucleus and parallel to the plane of the 

 future cleavage; these fibrils constitute the 

 (2) Close skein, or spirem. The chromatin 

 fibrils further thicken, be- 

 coming less convoluted, 

 and forming irregularly- 

 arranged loops, known as 

 the 



(3) Loose skein. The 

 question whether these 

 skeins are composed of the 

 contortions of one long 

 fibre, or whether they con- 

 tain several shorter ones, 

 has, as yet, not been defi- 

 nitely determined ; observa- 

 tions made on the cells of 

 lower forms, however, ren- 

 der it not improbable that 

 a single thread constitutes 

 the entire convolution. 

 The fibrils of the . loose 

 skein now separate at their 

 peripheral turns, so that a 

 number about twenty- 

 four (Flemming, Rabl) of distinct loops are formed; the closed 

 ends of these are directed towards a common centre, around which, 

 but removed some little distance, they become arranged. The 

 enclosed clear space is the polar field. During the formation of 

 the skeins the nuclear membrane disappears, its former position 

 being marked for some time longer by a clear zone or halo surround- 

 ing the nucleus and defining the boundary of the latter from the 

 cell-contents. Coincidently with the formation of the loose skein, a 

 very important phenomenon takes place. Within the achromatin 



Close skein, diagram of 

 nuclear fibrils : A, seen 

 from the side ; B, from the 

 polar field, P; C, from 

 anti-pole, GP. ( After Rabl- 

 Schiefferdecker.) 



Loose skein : nuclear spin- 

 dle has appeared in polar 

 field, P. (After Rabl-Schief- 

 fer decker.) 



