174 EMBRYO-SAC. [CH. XIII 



The embryo-sac and egg-cell. 



In a young ovule younger than that sketched in fig. 

 80 a single cell can be detected as differing in size and 

 appearance from its neighbours : this cell is called the 

 embryo-sac. The embryo is developed in its cavity, which 

 ultimately developes into a large hollow in the substance 

 of the nucellus. In fig. 80 the embryo-sac is shown as a 

 white space in the middle of the dark nucellus. The 

 embryo in fig. 82 lies in the embryo-sac, the limits of which, 

 however, are not shown. 



The nucleus of the embryo-sac undergoes a certain 

 process of division which leads to the state of things 

 shown in fig. 80, where a secondary nucleus has arisen, 

 together with certain other structures of even greater 

 importance. The primary nucleus divides into two 

 halves, and these halves again divide so that there come 

 to be four nuclei at one end of the embryo-sac, and 

 four at its other extremity. Two of the nuclei, viz. one 

 from each group of four, travel to the middle of the 

 embryo-sac and there unite to form the secondary nucleus 

 of the embryo-sac. One of the three nuclei remaining at 

 the micropylar end of the embryo- sac becomes the nucleus 

 of the egg-cell, while the other two form what are known as 

 the synergidce. The three nuclei at the opposite end of 

 the embryo-sac form a group known as the antipodal cells. 

 The last-named cells are of no further importance, the 

 interest now centres in the egg-cell, and in a much less 

 degree in the synergidae. 



To make the further history of the egg-cell clear, it is 

 necessary to return to the pollen-grain. The germination 



