II. The Ovule of St anger ia paradox a. 287 
can be followed through the middle of the nucellus towards 
the embryo-sac. 
No other disintegration or absorption of cells outside the 
sporogenous group has taken place. The sporogenous tissue, 
however, has undergone great changes with the increase in 
size of the megaspore. The latter is completely filled with 
the thin-walled parenchymatous tissue of the prothallus, the 
arrangement of the cells of which points to a gradual filling 
up of the cavity by divisions having taken place in a peri¬ 
pheral layer of cells. No starch is at this stage stored in the 
prothallus, which when fresh has a translucent appearance. 
Archegonia were not present, nor could the cells destined to 
form them be distinguished. The prothallus readily contracts 
during fixation, the wall of the megaspore which has become 
considerably thickened usually remaining more or less ad¬ 
herent to its surface. Around the megaspore a layer of cells 
was present which is clearly to be traced to the sporogenous 
group. The thick zone of sporogenous tissue present in 
earlier stages has, however, become reduced to a single layer. 
That this has taken place by the absorption of the cells 
between the outermost layer of sporogenous cells and the 
embryo-sac is indicated by the presence of smaller cells, in 
a more or less crushed condition, internal to the definite layer 
referred to. The latter appears thus to be derived from the 
most external layer of the sporogenous group, but material to 
follow the steps of the process was not available. The cells 
of this persistent layer (Fig. 16) are very large, and stand 
with the longer axis at right angles to the surface of the 
megaspore. Sometimes a single nucleus is present; often 
two are found in a cell. How long this layer of cells, which 
at this stage shows no signs of crushing or disintegration, 
persists, cannot at present be determined; in the fertilized 
seeds all trace of it was gone. Observations on aborting 
ovules showed that a similar increase in size of the outer 
layer of sporogenous cells takes place even when a normal 
embryo-sac is entirely wanting. In the light of the present 
facts it would appear to be a probable conclusion that, while 
