XXX PREFACE. 
consist first of an extension of the cavity abovemen- 
tioned, until with the exception of some few cases 
hereafter to be specified, this originally cellular thick 
solid body, is reduced to a thin coating, or even and 
not unfrequently reduced to nothing. 
It will be seen that not one of these phenomena 
depend upon the completion of fecundation, nor in 
fact does any one, excepting the all important forma- 
tion of the embryo, depend upon the due performance 
of this function. 
The 4th change consists in the production of a 
membrane within the cavity already formed in the 
nucleus. To this sac, I attach very great importance, 
and I consequently shall be full in my details. 
(Composite) whatever its nature may be, it appears 
to be always? formed from above downwards, that 
it is developed from the apex of the nucleary cavity: 
when completely developed, it invariably communicates 
with both apex and base of the excavation, the dia- 
meter being either uniform throughout, or larger at 
its apex, dividing by inferiorly into a thread of commu- 
nication. 
I know of no instance of the absence of this sac, 
of which from the consideration of Viscum and Santa- 
lum, I consider the only essential part of the ovula, 
preexistent to the embryo. 
There are two curious modifications of this integu- 
ment, and of great importance to study, and although 
the origin is in both cases the same, yet the growth is 
materially different, in the one, the albuminary form, 
consisting of the simple extension of a single cell, in 
the other, the exalbuminary form, of the ue d 
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