34 



INTRODUCTION. 



the aster is converted into a hollow cone whose apex is the centro- 

 sphere. Neighboring radiations unite and grow rapidly in length, at 

 the same time bending back toward the nucleus in a manner resem- 

 bling the spray from a fountain. An optical section of this stage is 

 shown in Fig. 12, C. With further growth the kinoplasmic rays give 

 rise to a sort of bell-shaped or half-ellipsoidal structure whose center 

 is occupied by the nucleus and whose pole is formed by the centro- 

 some (Fig. 1 2, D). Near the centrosome the fibers have already formed 

 a continuous but extremely thin layer, the plasma membrane, separat- 

 ing the cytoplasm of the spore from that of the ascus. At the edge of 



FIG. 12. Free cell-formation in ascus of Erysiphe communis. 



A, nucleus with centrosphere. 



B, development of nuclear beak. 



C, polar radiations extend outward and backward as spray from a fountain. 



D, formation of plasma membrane from end of beak outward, and continued growth of kinoplasmic 



fibers backward. 



E, F, meeting of fibers at opposite end of ellipsoidal spore and establishment of a complete plasma 



membrane delimiting spore-plasma from remaining plasma of ascus. (After Harper.) 



the bell the radiations end as free fibers, continuing their growth, how- 

 ever, in a direction corresponding to the periphery of the ellipsoid 

 (Fig. 12, E). Fipally these fibers meet in a point which is directly 

 opposite the centrosome, and unite end to end and laterally. The for- 

 mation of the plasma membrane continues, so that eventually an ellip- 

 soidal or oval cell is delimited from the cytoplasm of the ascus by 

 a complete plasma membrane (Fig. 12, F). At first the plasma 

 membrane is thicker near the centrosome, but later its thickness be- 

 comes uniform throughout. 



