740 Wisconsin Academy of Sciences, Arts, and Letters. 
These develop into multinncleated branches of the ascogone. 
The nuclei, however, are soon separated by cell walls, except 
in certain cases in which two nuclei are included in a single 
cell. These binucleated cells will later become the asci. 
Before the young asci are formed, the perithecium begins 
to show some differentiation in its hyphal layers. There is an 
outer layer of wide-lumened cells, already showing some thick¬ 
ening in their walls, on the upper side of the perithecium. 
Within are two or three layers of thin-walled cells, smaller 
and more densely filled with protoplasm. The inmost layer 
of these is especially active; it grows and sends branches in to¬ 
ward the center of the fruit body, crowding against the as- 
cogenous hyphae, intertwining among them and becoming di¬ 
vided to form the so-called “nurse cells.” The nurse cells are 
uninucieated or multinucleated, and are thus seen to have 
been formed from centripetal branches which are at first mul¬ 
tinucleated but are later cut up into smaller cells. This in¬ 
growth of the perithecial cells is practically the same as de¬ 
scribed by Harper for Erysiphe communis (11). Certain 
binucleated cells of the ascogenous hyphae are meanwhile de¬ 
veloping into asci. With their growth, the nurse cells are 
crowded back and flattened between the asci and the perithe¬ 
cial wall. Tangential sections of half-grown perithecia show 
these thin-walled cells as polygonal plates with two or more 
nuclei. 
The young asci when first recognizable are little larger than 
the other cells of the ascogenous hyphae. They present very 
irregular forms, probably due to the crowded condition within 
the perithecium at this time, but soon round out their angles, 
growing at the expense of the surrounding cells which they 
push back. The two nuclei in the young asci, although ly¬ 
ing very close together, at first show no tendency to fuse. 
They have well-defined centers to which the chromatin is at¬ 
tached, the strands extending back into the nuclear cavity in a 
typical cone (Fig. 5). The nucleole often lies near the nu¬ 
clear membrane opposite the center. This antipodal relation 
of center and nucleole is very common throughout the nuclei 
of Microsphaera. 
