442 Wilson . — Spermatogenesis in the Bryophyta . 
of a spermatid seen in a similar position to that shown diagrammatically in 
Text-fig. 2. Here the nucleus apparently occupies one end of the cell, and 
the thread passes about half-way along the flat side. In this case the two 
halves of the thread (seen to the left and right of i, i, Text-fig. i) are 
exactly superposed, and in consequence no change in position is noticed on 
altering the focus. Seen in one plane only, the thread should appear in 
section as a dot, but on account of the depth of focus possessed by the 
objective, this appearance is never realized. When the spermatid is seen 
in optical section along the line 2. 2 (Text-fig. i), the nucleus appears to 
occupy the whole of the cell, and the thread, on altering the focus slightly, 
apparently extends along the whole length of the flat side. Optical sections 
in the planes 3, 3 and 4, 4 (Text-fig. 1) produce appearances somewhat 
similar to that given along the plane 1,1. 
Optical sections in a plane making an angle of less than 90° with the 
plane of the diagram shown in Text-fig. 1 produce different appearances. 
Such will in future be referred to as oblique sections. The direction of 
a section passing obliquely through the line 1 , 1 (Text-fig. 1) is shown by 
1, 1 (Text-fig. 3). In such an optical section the outline of the spermatid, 
if examined at one focus, is bounded by one flat and one curved side as 
before. The nucleus apparently occupies one end of the cell, and the 
thread is seen as a slightly elongated structure at the end of the cell in 
contact with the nucleus. On focusing up and down the flat wall is 
apparently replaced by a slightly curved one, and the two halves of the 
thread (shown right and left of 1, 1 in Text-fig. 1) come into view, one 
occupying the slightly curved and the other the more strongly curved wall. 
Fig. 78, PI. XXXVIII, is a composite drawing made at several foci, and 
shows the appearances just described. In an oblique optical section taken 
through the line 2, 2 (Text-fig. 1), the direction of which is given by line 2, 2 
(Text-fig. 2), the nucleus apparently occupies the whole cell ; as before, the 
spermatid is bounded by one curved and one flat wall. At one focus the 
thread is seen as two short rods, one at each end of the cell ; on altering 
the focus the flat wall is apparently replaced by a slightly curved one, and 
the rods appear to approach one another along the more strongly curved 
wall. The total impression given is that the thread passes along the curved 
walk 
Fig. 76, PI. XXXVIII, shows the appearance of an oblique optical sec- 
tion through the line 3, 3 (Text-fig. 1), the direction of which is indicated by 
the lines 3, 3 in Text-figs. 2 and 3. Here the nucleus apparently occupies 
one end of the spermatid, while the blepharoplast is found near the opposite 
end. The thread appears to pass along the more strongly curved wall. 1 
1 The description just given has been written with the aid of a model. Without the assistance 
thus provided it was found to be extremely difficult to appreciate the space relationships in such 
a body as the spermatid of Pellia . 
