438 Wilson . — Spermatogenesis in the Bryophyta . 
found in the developing antheridium. When first distinguished, the centro- 
spheres are found lying in the cytoplasm at opposite ends of the nucleus, 
and no evidence as to their origin was obtained. According to J. and W. 
Docters van Leeuwen-Reijnvaan (42), a single centrosome is at first present 
in the cytoplasm of the spermatogenic cells of Pellia , and this by division 
produces the two found later at the spindle poles, but in spite of careful 
search, no structures resembling those figured by these investigators were 
found. 
Before the final division the spermatogenic cells shrink slightly and 
free themselves from their surrounding walls, becoming ellipsoidal or almost 
spherical in form. The walls swell considerably, so that each cell comes to 
be in an almost spherical space, separated by a considerable thickness of 
wall from the surrounding cavities. Each possesses a large nucleus which 
extends over more than half the cell and contains a well-marked nucleolus. 
The early stages of division resemble those already described (PI. XXXVIII, 
Fig. 66). Small deeply staining granules were sometimes seen near the 
pointed ends of the elongated nucleus, but owing to their close proximity to 
the periphery of the cell, their presence could not always be determined ; 
radiations could not be distinguished for the same reason. A little later the 
nucleus increases in size, and the pointed ends come into contact with the cell 
membrane (Fig. 67). The chromatin is now found, in a threadwork which 
is arranged in an equatorial band just within the nuclear membrane, while 
the remainder of the nucleus is free from staining substance. The chromo- 
somes then become distinct, and are found scattered throughout the central 
part of the protoplasm. At the next stage discovered, the daughter chro- 
mosomes had almost reached the poles, and since here only an extremely 
thin layer of cytoplasm exists between the ends of the spindle and the cell 
membrane, no bodies resembling centrosomes could be distinguished. 
After their arrival at the poles the chromosomes become crowded 
together and lose their sharp outlines, finally forming a number of irregular 
deeply staining masses. These are pressed closely against the periphery of 
the cell, and in consequence it is impossible to see whether centrospheres 
are present at this stage (Fig. 68). Meanwhile, thickenings of the spindle 
fibres form at the equator, and a little later by coalescence give rise to 
a wall separating the two daughter-cells or spermatids. At the same time 
retrogressive changes have gone on in the daughter nuclei, and a nuclear 
membrane appears ; no definite nucleolus is present, but each contains 
a number of deeply staining, irregular masses of chromatin. 
Owing to the circumstance that the mother-cell is almost spherical, 
each spermatid is approximately hemispherical in form, and in the prepara- 
tions appears either circular or semicircular in outline, depending on 
the direction from which it is observed. In the following description the 
spermatid, when seen in section as a semicircle, is spoken of as viewed from 
