II. The Cytology of the Gametophyte Generation. 185 
to temperature and light prevailing at the time, and the period of the year. 
The whole process is gone through with considerable rapidity, and all the 
stages may be found in a single sorus. In the case of the tetrasporangium 
the stalk-cell separates while the rudiment is still comparatively small ; 
here, however, it is delayed till the cell is nearly fully grown, and liberation 
follows in about three to five days. 
Of the oogonia in a sorus the central ones only are vertical to the 
thallus ; in consequence of their great increase in diameter as compared 
with the stalk-cells the others lean outwards, the angle being greater as 
the periphery is approached ; this makes it difficult to obtain median 
sections through the dividing nuclei. 
It is generally stated that there are no borders of elongated sterile cells 
to the oogonial sori, as there are to the antheridial ones. This, however, 
is not strictly correct ; borders, partial or complete, are frequently found 
and may arise in one of two ways. 
1. Some of the outer cells of a sorus may at a comparatively early 
stage in their development be compressed by the more actively growing 
inner ones, and prevented from developing further. These are visible in 
sections as narrow elongated cells, but in a surface view are completely 
hidden by the outermost of the fertile oogonia. 
2 . Towards the close of the season, when unfavourable conditions 
supervene, most of the outermost oogonia fail to mature even after attaining 
their full size; the cytoplasm is smaller in amount and the nucleus does 
not divide. These facts suggest the mode in which the antheridial borders 
may have arisen, and the manner in which borders may also in the course 
of time be acquired by the oogonial sori. In this connexion it is instructive 
to notice that the central portions of both male and female sori are more 
active than the peripheral ones ; in the early stages the nuclei are more 
advanced, liberation generally starts in the middle of the sorus, and sterili- 
zation, although it sometimes occurs there, is not nearly as frequent as 
it is in the marginal regions of the sori. 
Fig. 1 shows the structure of the oogonium-rudiment shortly before 
the separation of the stalk-cell, as seen in a section vertical to the surface 
of the thallus and transverse to the branch. As in the case of the tetra- 
sporangium-rudiment the basal part of the cell has very large vacuoles, 
while the remainder is occupied by denser cytoplasm, in which the chloro- 
plasts are much more crowded. The curved, rod-like centrosomes and 
radiations are very distinct, not only at the distal pole but most commonly 
at the basal pole as well. The physodes as usual are chiefly located in 
the peripheral layers of the free surface. 
The early prophase stage is very much like that of the stalk-cell 
division in the tetrasporangium ; there is the same coarse chromatin 
thread, irregular in the arrangement and thickness of its granules, and soon 
