i6 



NORMAL HISTOLOGY. 



FIG. 



FIG. 8. 



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

 Schleicher, Rabl, and others have added much to the accurate 

 knowledge of the life-history of the cell. 



The entire process of indirect cell-division may be divided con- 

 veniently into three stages, which, however, gradually follow one 

 another without sharp demarcation. These stages are : (i) the Pro- 

 phases, embracing the preparatory changes of the nuclear constitu- 

 ents ; (2) the Metaphase, during which the 

 accurate division of the chromatin is effected ; 

 (3) the Anaphases, in which distribution and 

 rearrangement of the newly apportioned chro- 

 matin takes place. Coincidently with the com- 

 pletion of the new nuclei, the cell-protoplasm 

 undergoes division, so 

 that each daughter - cell 

 consists of the new nu- 

 cleus surrounded by a 

 definite area of proto- 

 plasm. These final 

 changes constitute the 

 Telophases. 



When the cell under- 

 goes a complete and typi- 

 cal mitotic division, the 

 following changes occur : 

 (i) The centrosome in- 

 creases in size and, if not 

 already existing double, 

 divides into two ; the nu- 

 cleus enlarges, while the 

 chromatin greatly in- 

 creases, the fibrils becom- 

 ing contorted to form a 

 dense convolution, whose 

 twisted threads run gener- 

 ally 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, 

 becoming less convoluted, and forming irregularly-arranged loops, 

 known as the 



(3) Loose skein. The skeins, sometimes composed of the con- 

 tortions of one long fibre, at others of several shorter ones, separate 

 at their peripheral turns, so that a definite number of distinct loops, 

 or chromosomes, are formed ; the closed ends of these are directed 



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- 

 ferdecker.} 



