152 - DEVELOPMENT OF THE OVULUM 
very rarely visible. In the ripe seed it is distinguishable 
from the inner membrane only by its apex, which is never 
perforated, is generally acute, and more deeply coloured, or 
even sphacelated." 
M. Brongniart specified Thesium as an instance, and sus- 
pects that a similar formation will be found in all Santalaceze. 
I am able to corroborate this so far as Santalum itself is con- 
cerned, and my friend M. Decaisne informs me, that such is 
his view of the ovula of Helwingia. M. Mirbel is however, I 
believe, opposed to the idea, at least to that of M. Brongniart. 
I believe it will be found that the statement is correct, and 
I may observe that the difference between a really naked 
nucleus, and one which is.united intimately to its coat, is very 
distinct, although not easily to be expressed in words. 
The nucleus undergoes no important special change until 
at or about the period of fecundation. 
The first indication consists in an appearance of opacity 
visible generally towards its centre, and which appears to be 
the prelude to the formation of excavation. 
'The formation of the cavity, which is antecedent to the 
appearance of the embryonary sac, generally, I think, com- 
mences from below upwards. It is carried on to a grea- 
ter or less extent, but in all cases, excepting Santalum and 
those in which albumen is deposited in the nucleary tissue, 
this organ becomes at length reduced to a thin coat. And 
in this form it will in most cases be distinguished in the ripe 
seed. 
For the membrane resulting from the excavation of the 
nucleus, M. Brongniart proposes the name of membrane a 
ispermique. 
In his view of the nucleus, M. Brongniart agrees with 
Mr. Brown, except in the mamelon d' impregnation. Some- 
times however, M. Brongniart, if I understand him rightly, 
appears to be of opinion, that in all cases the apex of the 
nucleus is exposed at the time of fecundation, even in those 
(as Compositæ), in which it coheres with the coat, 
Subsequently, however, he expresses himself clearly to this 
effect: nous avous vii que l' amande se presente sous la forme 
