32 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



chromatin is in intimate connection with the central body (figs, i, 3, 4, 

 5, 7), and in specially favorable preparations (figs. 4, 7) it is clear that 

 there are strands in the chromatin reticulum which radiate back from 

 the central body into the cavity of the nucleus, forming an intranuclear 

 system almost like an aster whose rays appear quite straight and definite 

 in many cases, though they are connected by transverse fibrillse and lose 

 themselves in a more indefinite granular reticulum in the region oppo- 

 site the central body. The nucleus has plainly, at this stage, a unipolar 

 structure, and the chromatin is definitely oriented with reference to the 

 central body. Following Rabl's terminology, I shall speak of that part 

 of the nucleus which is antipodal to the central body as the antipolar 

 region and the region about the central body as the polar region. 



The nucleus of the oogonium has regularly a single oval or spher- 

 ical dense homogeneous nucleole (figs. 1-7), which commonly lies some- 

 where in the antipolar region, but may frequently lie close to the central 

 body, crowding aside the chromatin elements. 



The nucleus of the antheridial cell is still smaller prior to fusion 

 than that of the oogonium ; still its central body is sharply differentiated 

 and the chromatin is plainly connected with it (figs. 5, 6). It has also 

 regularly a single nucleole. The structure of the nucleus of the anther- 

 idial stalk-cell and of the other vegetative nuclei agrees with that of the 

 sexual cells (figs. 4, 5, 7). Frequently the hyphal nuclei are oval or 

 more elongated, and may be drawn out to a point on which the central 

 body sometimes lies. (Compare figs. 24 and 14) . After its migration 

 into the oogonium the antheridial nucleus enlarges and the two pro- 

 nuclei are each plainly seen to be provided with central bodies at a stage 

 just before they fuse (fig. 9)., The fertilized egg-nucleus also shows 

 a single conspicuous center (figs. 10, 11, 12), and it seems probable that 

 it has been formed by the combination of the centers of the pronuclei. 

 It is also probable that this central body of the fertilized egg- 

 nucleus is actually double as compared with those of the pronuclei, since, 

 as we shall see later, we have strong evidence of the permanence of the 

 chromosomes as cell structures, and a nuclear fusion should hence pro- 

 duce a nucleus with a double number of chromosomes, each of which 

 would have an independent attachment to the central body. But I have 

 not been able to obtain a sufficient series of preparations at this stage 

 to be able to trace the process of the combination of the chromatin and 

 centers in this fusion, nor to determine the number of chromosomes in 

 the next succeeding division. The fertilized egg-nucleus regularly con- 

 tains a single, rather large nucleole (figs. 10, 12), which, it seems prob- 

 able, is also formed by the fusion of the nucleoles of the pronuclei. 



