50 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



while in contact with the plasma membrane of the ascus some of the 

 material of the latter may pass over into the rays and thus aid in forming 

 the membrane of the spore. There is, however, no break in the plasma 

 membrane of the ascus as a result of any such possible participation 

 in the formation of the new spore membrane. The membrane of the 

 ascus remains as a continuous envelope of its cytoplasm, and there is 

 no apparent loss of turgidity in the latter. In Phyllactinia also it is 

 quite common to find the cytoplasm very loose and showing large vacu- 

 oles in the neighborhood of the beaked nuclei (figs. 70, 72). Th'ese 

 spaces may represent the remains of the nuclear cavity of the preceding 

 mother nucleus. This looser cytoplasm is sharply bounded by the inner 

 rays of the aster (figs. 70, 72). 



The stages in the process of the folding over of the rays and their 

 union to form the plasma membrane of the spore are very well shown 

 in B. cichoracearum. The rays become elongated during the process 

 by growth which apparently proceeds from the central body outward, 

 and at the same time they fold over and combine side by side to form a 

 continuous broad, umbrella-shaped membrane (figs. 74, 81). Sometimes 

 the rays on one side seem to be in advance of those on the other in the 

 process of inclosing the spore-mass (fig. 73). If, in folding over and 

 elongating, the rays of one center come in contact with those of another, 

 they tend to fuse, at least temporarily (fig. 74) . Later, however, they 

 must separate again, since one almost never finds spores with two nuclei, 

 while such conditions as those shown in fig. 74 are not uncommon. 



That the rays actually combine to form a membrane in these early 

 stages is shown in Phyllactinia, as in B. communis, by the fact that the 

 polar region of the spore may, draw away from the adjacent cytoplasm 

 as a result of fixation before the spore is entirely delimited. The broad, 

 umbrella-shaped membrane shown in figs. 74 and 81 gradually closes in 

 to form, by further margiijal growth, the ellipsoidal plasma membrane 

 of the spore (fig. 75). ^he whole spore body is cut out of the previ- 

 ously undifferentiated cytoplasm of the ascus by the formation of a new 

 plasma membrane derived from the fibers of the polar aster and without 

 the deposition of a cellulose wall. In this case, as in animals and the 

 higher plants, the process of cell division consists in the formation of 

 new plasma membranes which, in the latter at least, originate as a so- 

 called cell plate from the fibers of a portion of the karyokinetic figure. 



Such a process of migration of the astral fibers demands the 

 assumption that they are contractile elements comparable to cilia, even 

 though we have not as yet sufficient data on which to carry out such a 

 comparison in every detail. The abundant evidence which has accu- 



