36 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



the pole. In the prophases of the division of the nuclei in the asci of 

 M. communis the connection frequently appears partially ruptured (38, 

 Taf. II, figs. 4, 11). This may be due to the fixation. Occasionally 

 the undifferentiated portion of the chromatin reticulum rests also on 

 the nuclear membrane over the whole antipolar region, and this may 

 possibly be regarded as the more normal condition. The nuclear mem- 

 brane is spherical in such cases, and the only region of clear nuclear 

 sap is that left by the drawing together into a cone of the chromatin 

 threads which are converging on the central body. 



In some cases the zone of clear nuclear sap extending around the 

 entire chromatin system, except at the pole, is so wide as to suggest 

 that the nuclear membrane has swelled away from the nuclear contents 

 in fixation. Such figures, assuming that they indicate swelling of the 

 nuclear membrane, give evidence of the firmness of the attachment of 

 the chromatin threads to the central body and of the latter to the cyto- 

 plasm, since in well-fixed material the chromatin is never separated 

 from the center nor the latter from the cytoplasm, no matter how wide 

 the clear zone may be about the remainder of the chromatin system. 



Sometimes also a clear zone is formed about the nucleus outside 

 the nuclear membrane, such as has been described for the nuclei of 

 Chara; but in this case the central body is neither separated from the 

 nuclear membrane nor the cytoplasm, indicating again its connection 

 with both of those structures. Not infrequently the chromatin threads 

 at this stage show parallel bends and curves, and I find many figures in 

 which the whole system tends to be slightly spirally twisted. There is 

 also some evidence that the threads are arranged in pairs. 



When the young ascus ha% reached the stage shown in fig. 33, the 

 pair of nuclei fuse to form the primary nucleus of the ascus. The 

 process of fusion can be followed in all its details with relatively great 

 readiness in Phyllactinia, owing to the large number of asci in each 

 perithecium. At the time of fusion the ascus-cell consists of an upper 

 enlarged portion, in which the nuclei lie, and a lower stalk-like portion, 

 which is much narrower and extremely irregular in its shape, twisting 

 about among the wall-cells of the base of the perithecium and connecting 

 with the original system of the ascogenous hyphse from which it devel- 

 oped. A section of the upper portion of the ascus rarely shows this 

 elongated narrower portion in its entire length, since the two rarely lie 

 in the same vertical plane. Later, with the growth of the perithecium, 

 this stalk-like portion of the ascus swells and becomes a part of the 

 oblong ascus-sac, except at its lowest portion, which still remains as a 

 narrowed foot or stalk. 



