OTHER MODES OF NUCLEAR DIVISION 



209 



The two kinds of chromatin now separate, the "trophochromatin placing 

 itself in the center and the generative or idiochromatin lying like a thin 

 equatorial plate around it." As the nucleus elongates the tropho- 

 chromatin body becomes dumbbell-shaped and breaks into two, while 

 the idiochromatin plate splits into daughter plates which apparently 

 move to the poles and cooperate with the trophochromatin in the forma- 

 tion of the daughter nuclei. 



FIG. 74. 



A, two stages of mitosis in Sorodiscus. (After Winge, 1912.) B, anaphase of nuclear 

 division in Euglena. Chromosomes grouped about dividing "nucleolo-centrosome." 

 (After Keuten, 1895.) C, chromosomes developing from nucleolus in Spirogyra. X 1335. 

 (After Berghs, 1906.) D, Mitosis in Spirogyra crassa. (After Merriman, 1913.) 



A process with much the same appearance at certain stages is seen in 

 the flagellate, Euglena (Keuten 1895) (Fig. 74, B). Here the chromo- 

 somes group themselves about the large nucleolus which soon takes the 

 form of a dumbbell-shaped "central spindle" or "centrodesmose." 

 The nucleolus completes its division, the chromosomes meanwhile 

 separating into two groups which pass to the poles and reorganize the 

 daughter nuclei. In certain other flagellates Kofoid (1915) reports a 

 split spireme and a definite number of chromosomes which differ markedly 

 in size and shape. 



In Cladophora (Carter 1919) nearly all the chromatin is contained in 

 one or more large chromatin nucleoli, or karyosomes. After the numer- 

 ous chromosomes have arrived at the two poles at the close of the ana- 

 phase the spindle connecting the two groups constricts and completes 

 the division of the nucleus. 



Another unusual condition is found in Spirogyra (Fig. 74, C, D). 

 In this form nearly all of the chromatic material is lodged in the large 

 nucleolus, the nuclear reticulum being very delicate and almost invisible 

 in many preparations. According to Berghs (1906), Karsten (1908), 

 and Trondle (1912) all the chromosomes which appear in the prophase 

 and split as usual are derived from this nucleolus, most of its material 



14 



