
180 DEVELOPMENT OF THE OVULUM 
All the subsequent changes at least those of any import- 
ance are carried on within the nucleus. They consist first of 
an extension of the cavity above mentioned, until with the 
exception of some few cases hereafter to be specified, this 
originally thick, solid, cellular body, is reduced to a thin coat- 
ing, often so thin as to be with difficulty detectable. 
'The cavity thus formed will be found to present at its 
base or apex, a vesicular body which enlarges rapidly, and 
soon lines the whole of the cavity, and tends by a continuance 
- of enlargement to reduce the nucleus to the thin coat above 
mentioned. This sac soon becomes more or less filled with 
lax tissue, which subsequently is either absorbed or converted | 
into albumen. 
'The last change, and the only one actually dependent on 
fecundation, consists in the development of the embryo, which 
invariably takes place at that part of the embryonary sac 
nearest to the apex of the nucleus, at least when the de- 
velopment is carried on within this body. 
Before concluding the detailed consideration of the parts 
forming an ovule, I must observe that M. Mirbel and Mr. 
Brown differ as to the mode of formation of the teguments. 
M. Mirbel says, in his lst Memoire, p. 4. “dans l'origine, 
l'ovule n'est qu'une petite excroissance pulpeuse qui ne pa- 
rait avoir aucune envelope, aucune ouverture. Peu aprés, le 
point culminant de la petite excroissance se perce, et l'on 
commence à distinguer l'exostome, et l'endostome, et à la 
faveur de ces deux orifices, la primine, la secondine, et la 
nucelle.” It is rather difficult to comprehend the precise 
meaning of this passage, Mirbel says likewise, that the pri- 
mine is formed first, and he figures it as being so ; see his 
Mem. pl. 1, fig. 3, Cucumis Anguria: this is in my mind, 
the inner tegument, or his secundine, at least to reconcile it 
with the excellent figures given by the same author. Mr. 
Brown's explanation is much more conformable to what actu- 
ally takes place, and I have adopted it without hesitation. 
I have before alluded to a change of situation frequently 
occurring in the ovulum in its earlier stages of growth. For 
a due knowledge of this we are indebted to M. Mirbel. 

