GAMETOPHYTE AND SPOROPHYTE. 



24I 



473. The chromatin skein. — One of the first nuclear figures 

 in the preparatory stages of division is the chromatin skein or 

 spirem. The chromatin substance unites to form this. The 

 spirem is in the form of a narrow continuous ribbon, or band, 

 woven into an irregular skein, or gnarl, as shown in figure 316. 

 This band splits longitudinally into two narrow ones, and then 

 each divides into a definite number of segments, about eight in 

 the case of podophyllum. Sometimes the longitudinal splitting of 

 the band appears to take place after the separation into the chro- 

 matin segments. The segments remain in pairs until they separate 

 at the nuclear plate. 



474. Chromosomes, nuclear plate, and nuclear spindle. — 

 Each one of these rod-like chromatin segments is a chromosome. 



Fig. 318. 



Karyokinesis in pollen mother cells of podophyllum. At the left the spindle with the 

 chromosomes separating at the nuclear plate ; in the middle figure the chromosomes have 

 reached the poles of the spindle, and at the right the chromosomes are forming the daughter 

 nuclei. (After Mottier.) 



The pairs of chromosomes arrange themselves in a median plane 

 of the nucleus, radiating somewhat in a stellate fashion, forming 

 the nuclear plate, or monaster. At the same time threads of the 

 protoplasm (kinoplasm) become arranged in the form of a spindle, 

 the axis of which is perpendicular to the nuclear plate of chromo- 

 somes, as shown in figure 318, at left. Each pair of chromosomes 

 now separate in the line of the division of the original spirem, 

 one chromosome, of each pair going to one pole of the spindle, 



