4O SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



fringe. It is plainly identical with the fibrillar material of the earlier 

 stages. This contracted stage lasts for some time if estimated in terms 

 of the growth of the ascocarp. 



While the chromatin is thus drawn up to the centers the outlines 

 of the latter are difficult to make out in preparations stained so as to 

 give the chromatin a deep-blue color. The whole chromatin mass in 

 deeply stained specimens may appear as a roughly oval body pressed 

 against the nuclear membrane in the region of the central body. If, 

 however, the staining be modified by increasing the time of exposure 

 to the orange G of the triple stain, until the blue is removed from the 

 chromatin, leaving it a pale-gray, the center appears with its customary 

 disk-like shape and showing a dense violet color. With this treatment 

 the red nucleole and the violet center are the only deeply stained por- 

 tions of the entire ascus. The chromatin is pale gray and the cytoplasm 

 faintly gray or orange. The appearance of such a preparation is shown 

 in fig. 45. A similar sharp differential staining of the center can be 

 achieved by the use of iron hsematoxylin, the washing out with the iron 

 solution being prolonged till the chromatin is colorless. Both nucleole 

 and center appear bluish or black with this treatment. Preparations 

 made in this way show the persistence of the center during the con- 

 tracted condition of the chromatin and demonstrate very clearly the 

 possibility of differential staining of the center by appropriate methods. 



It is clear that this contracted stage of the chromatin elements is 

 identical with the synapsis stage in the spore mother cells of the higher 

 plants. Here, as there, it is probably associated with an interaction and 

 combination of the chromosomes prior to reduction, and the evidence 

 from the attachment of the chromatin threads to the center is practi- 

 cally conclusive that the combination occurs by the union of the chro- 

 matin threads in pairs side by side. We shall find that the succeeding 

 spirem stage shows chromatin strands of unusual thickness and density. 



The contracted condition of the chromatin is followed by a loosen- 

 ing up of the mass and a transition to a very strongly marked spirem 

 stage in the preparation for the first division. As the chromatin mass 

 spreads out again into the antipolar region, and loosens up, it frequently 

 appears for a time as if reticulated (fig. 46). Very soon, however, the 

 threads become more distinct, their apparent anastomoses largely disap- 

 pear, and they form an irregular cone, with its apex in the central body 

 (fig. 47), such as we found prior to the nuclear fusion. The threads 

 at this stage are, however, much more distinct and sharply outlined than 

 in the earlier stages. There also seem to be fewer of the faintly stained 

 interfilar fibrillse. It should be noted that all the nuclei in the phase of 



