8 



INTRODUCTION. 



The ascus of this species offers unusually favorable material for the 

 study of mitosis on account of the clearness with which all details are 

 brought out, and because the three successive nuclear divisions follow 

 each other rapidly, making it possible to trace with unmistakable 

 clearness the persistence of the centrosome from one nuclear genera- 

 tion to the other. Since the spindles lie in different planes, it is pos- 

 sible also to observe, side by side, the same stages at different angles 

 in the same field of the microscope. The following refers especially 

 to the second mitosis in the ascus. 



FIG. 4. Mitosis in ascus of Erysiphe communis. (After Harper.) 



A, nucleus in resting stage of second nuclear generation in ascus, the flattened or disk-shaped centro- 



some closely applied to nuclear membrane. 



B, early prophase ; the kinoplasmic radiations have been developed about the centrosome. 



C, D, E, F, successive steps in development of spindle. 



G, mature spindle, the nuclear membrane still persists at sides. 



H, end of anaphase ; connecting fibers extend between the daughter nuclei, which are not yet provided 



with a nuclear membrane. 



I, daughter nucleus provided with membrane, kinoplasmic radiations present. 

 J, later stage in which the polar radiations have disappeared. 



Between the successive nuclear divisions in the ascus, the chromatin 

 of the daughter nuclei does not assume the complete resting condition. 

 It consists (Fig. 4, A) of an irregular net with the angles of the 

 meshes somewhat thickened. Generally the net lies tolerably free in 

 the nuclear cavity, and a very distinct nucleolus is present. The 

 centrosphere appears as a flattened disk closely applied to the nuclear 

 membrane, giving the impression as if the two were grown together 

 (Fig. 4, A). The chromatin net appears also attached at this place 



