38 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



pass up or down from the center so as not to lie in the median plane of 

 the nucleus as a whole. There are then, approximately at least, eight 

 or nine threads in each system just before fusion occurs. The exact 

 number, of course, is difficult to determine in every case, since in such 

 minute objects it is difficult to be sure in focusing up and down that 

 different threads are not confounded with each other in certain portions 

 of their extent, or that some threads do not escape notice altogether 

 by being hidden behind others lying in nearly the same vertical plane. 



The two centers can be found closer and closer together, and finally 

 they combine side by side, forming at first a double center, from which 

 the distinct sets of fibers can be still made out (fig. 38) . A little later 

 the two sets of fibers are no longer distinguishable (fig. 39). The 

 centers at the time of fusion maintain their ordinary disk-like shape, 

 and it is plain that the fusion figures will have a diflferent appearance 

 according as the centers lie side by side or one above the other. The 

 former are, of course, much more favorable for study. 



The nucleoles also fuse at some time during the process of the 

 fusion of the nuclei. They are extremely conspicuous, bright-red glob- 

 ules, dense and homogeneous, and most sharply distinguishable by their 

 red color in the triple stain from the blue-stained chromatin. Each 

 nucleus contains without exception a single nucleole, and when the 

 nuclear membranes disappear in the process of fusion the two may be 

 very variously placed, as shown in the figures. Later they approach 

 or are brought together in the movements of the other parts of the 

 nucleus, and when once in contact they remain together and gradually 

 fuse into a single nucleole whose diameter is conspicuously greater than 

 that of either of the two which fuse. (Compare figs. 31-33 with 36, 37, 

 39, 42.) If they had a perfectly spherical shape it would, of course, 

 be easy to determine the exact relation of the voliimes of the two which 

 fuse with that of the resulting nucleoles. They are, however, frequently 

 somewhat oblong or flattened on one side, and since it is not easy to 

 determine their vertical diameters in any given case, exact measurements 

 of their volume can not be obtained. As is seen from figs. 36-42, the 

 fusion of the nucleoles may be completed either before or after the fusion 

 of the centers. The nucleoles at this stage always lie outside the chro- 

 matin systems and their fusion seems to be an entirely distinct process. 



The process of nuclear fusion may be summed up as a union of the 

 two resting nuclei into a single spherical nucleus whose volume is much 

 greater than that of either of the single nuclei, whether or not it is 

 exactly equal to their sum. The nuclear sap of the two makes a single 

 homogeneous non-stainable liquid. The centers fuse into a single larger 



