4 8 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



tion. In this upper spindle the chromosomes are a little nearer the 

 poles than in the lower one, but in both either seven or eight chromo- 

 somes can be made out in each group. 



Figs. 67 a, b show three nuclei from an ascus in the four-nucleated 

 stage. They are all in the stage of division when the daughter chro- 

 mosomes are just separating out of the equatorial plate. Fig. 67 a 

 shows one profile and one polar view of the spindles which lie at the 

 upper end of the ascus and are destined to form supernumerary nuclei. 

 Fig. 6;;' b shows a somewhat larger spindle [figure which lies near the 

 center of the ascus. It is probable that both nuclei formed from it will 

 become centers for spore formation. In this division also the chromo- 

 somes can be counted with perfect certainty as they are drawn back to 

 the poles. Fig. 68 shows two nuclei at this stage, and in each of the 

 four daughter groups seven or eight chromosomes can be counted. 



In both the second and third divisions it is plain that the central 

 body continues in the same relation to the chromatin as in the first 

 division and the fusions which preceded it. The figures in the two and 

 four nucleated stages are not so favorable for counting the number of 

 strands in the spirem stage of the prophases by reason of their greatly 

 diminished size, but as to the main fact, that the central body maintains 

 a continuous connection with the chromatin, just as in the first division, 

 the evidence is perfectly conclusive. 



As noted above, only two spores are formed as a rule in the asci 

 of Phyllactinia. This leaves six supernumerary nuclei which disinte- 

 grate in the epiplasm. Generally these supernumerary nuclei at the 

 time of spore formation are all in the peripheral end of the ascus, while 

 the two nuclei which are to become the centers of the two spores are 

 rather above the middle of the ascus. The supernumerary nuclei fre- 

 quently are pressed against the wall of the ascus, with their central 

 bodies on the side next the wall. Later several of these supernumerar)' 

 nuclei are frequently found lying in a bunch free in the cytoplasm. 



The process of spore formation is especially well shown in the asci 

 of Brysiphe dehor ac ear um, and I have included for these stages some 

 figures from this species with those from Phyllactinia. Figs. 80 and 81 

 are from B. communis. The polar aster from the third division persists 

 in all the eight nuclei for some time, but is most conspicuous in the case 

 of the nuclei which are to be inclosed in spores (fig. 69). A beak is 

 next pushed or pulled out from the nucleus, which is generally already 

 pear-shaped, as in the corresponding stages in the earlier divisions 

 (figs. 70, 71,72). 



