STRUCTURE.] AMNIOTIC SAC. 403 



that the ovule of Santalum album consists of nothing more 

 than a naked nucleus, from within the apex of which the sac 

 of the amnios protrudes in the form of a long tubular process. 

 The same excellent botanist considered it probable that it 

 will hereafter appear that the sac of the amnios is the only 

 essential part of an ovule. (Linnaan Transactions, xviii. 77.) 

 These observations were followed by others on the part of 

 M. Decaisne, who found that in Thesium the structure of the 

 ovule is of the same nature as that of Santalum. (Comptes 

 rendus, viii. 203.) 



The history and functions of this embryonic sac are so 

 curious and important that I find it necessary to give Mr. 

 Griffith's views at some length, although greatly abridged, 

 from his elaborate paper in the Linnaan Transactions : 



" The first protrusion of the amniotic sac beyond the apex 

 of the nucleus takes place long before the opening of the 

 flowers or anthers ; at the earliest period it presents itself as 

 a membranous tube of nearly equal diameter, the exserted 

 part being rather longer than the ovule; and the included 

 part apparently originating from the base of the ovule, 

 beyond which it does not appear to be extended posteriorly. 



When the flower-bud is half developed, the embryo-sacs 

 will be generally found to have attained nearly their full 

 length in regard to the outside of the placenta; and with 

 reference to the inside of this organ, they have undergone a 

 remarkable modification, consisting in their extension back- 

 wards and upwards, beyond the base of the ovule towards the 

 axis of the placentae. The apex of the sac appears up to this 

 period to be quite simple. An enlargement of the part near 

 the apex of the nucleus has commenced, and is called the 

 bulb or bulbous portion. 



The changes that occur in the sac, prior to fecundation, 

 consist in the deflection of the posterior extension of the sac 

 in the direction of the axis of the placenta, and in the appear- 

 ance of cellularity and division of the apex. The contents 

 likewise appear to undergo some changes; but these are 

 limited to the apex, and seem to consist of a sort of conden- 

 sation, and irregular and variable division, of the grumous 

 matter. At the period of expansion of the flower, the posterior 



