THE NUCLEUS 



85 



forms a new blepharoplast from its centrosome, and subsequently 

 a new flagellum ; or the blepharoplast of the parent cell divides 

 independently to form the blepharoplasts of the daughter-cells 

 (Fig. 43). In this last type, the blepharoplast, though obviously 

 a body of centrosomic nature, acquires a more or less complete 

 independence of the definitive centrosome, and becomes a distinct 

 cell-organ, permanent for at least a certain number of cell-genera- 

 tions ; it may multiply and undergo various structural complica- 

 tions, to be described presently. 



FIG. 42. Stages in the division of Spongomonas uvella. A, Resting condition of 

 the cell ; two flagella arise, each from one of a pair of blepharoplasts (diplo- 

 some) ; the nucleus contains a large karyosome, in which the centriole is 

 lodged, and a few irregular grains of peripheral chro matin in the nuclear cavity. 

 B, Early stage of mitosis ; an achromatinic spindle is formed with the centrioles 

 at the poles, one centriole (on the right) having already divided into two ; 

 the chromatin, both peripheral and central, has united to form a dense 

 equatorial plate in which separate chromosomes cannot be discerned ; the 

 flagella have disappeared, together with their blepharoplasts. C, Similar 

 stage in which the daughter-flagella are growing out precociously from the 

 centrioles, one on the left, two on the right. D, Later stage in which the 

 equatorial plate has split into two daughter-plates, but no flagella have as 

 yet grown out from the centrioles, of which there are two at each pole. E, 

 Division of the nucleus nearly complete ; no flagella. F, Nucleus completely 

 divided, daughter-nuclei in process of reconstruction ; from each a pair of 

 blepharoplasts has been budded off, still connected by a centrodesmose with 

 the centriole contained in the karyosome ; a pair of daughter-flagella has 

 arisen from one pair of blepharoplasts, but not as yet from the other. After 

 Hartmann and Chagas (62), magnification about 2,400. 



3. In certain flagellates- for example, trypanosomes and allied 

 forms (" Binucleata ") the cell-body contains two nuclei, as already 

 noted : a trophonucleus and a kinetonucleus. To what extent 

 these nuclei are provided with centrosomes is at present a little 

 doubtful ; probably this point is one which varies in different cases 

 (compare Wenyon, 84). There are, however, three chief possi- 

 bilities : (a) There may be but a single centrosome, that of the 

 kinetonucleus, which acts both as blepharoplast and division-centre 



