IOWA ACADEMY OF SCIENCES. 213 



embryo sac, ready for fertilization, measures approxi- 

 mately 52 u in length and 22 u in width, and occupies 

 much less than one-third of the length of the nucellus in 

 the one to the four-celled archisporial stage. There are 

 generally about two layers of cells of nucellar tissue 

 between the archisporium and the dermatogen of the 

 nucellus (Fig. 3-6). From the four-celled archisporium to 

 the two-celled embryo sac there is generally one layer of 

 cells between the macrospore and the dermatogen, and by 

 the time the embryo sac has reached the four-celled stage, 

 the tapetal end of it is in close connection with the derma- 

 togen, there being no tissue between the two. 



At about the time the pollen tube enters the egg cell one 

 of the synergids disappears. The other one remains 

 apparently unchanged until the process of fertilization is 

 completed, after which it is no longer present. The fusion 

 of the generative pollen nucleus with the egg cell and the 

 fusion vegetative nucles with the endosperm nucleus seem 

 to occur at about the same time. The fertilized egg cell, 

 before any division takes place, measures about 33 u long 

 by 14A u wide. The endesperm nucleus divides once 

 before the first division of the egg nucleus takes place; 

 the first division of the endosperm being in the direction 

 of the long axis of the embryo sac. The second division 

 is at right angles to the first, and it occurs at or just before 

 the time that the egg nucleus divides the first time. The 

 third division occurs in the lower two cells, resulting from 

 the second division, and also occurs in the same direction 

 as the second (Figs. 15, 16). The upper cells resulting 

 from the first division do not divide until two or three 

 divisions have taken place in the lower cells. But by the 

 time the embryo has reached the four-celled stage the 

 endosperm has extended w^ell up along the side of the 

 embryo (Fig. 17). 



The first division of the egg cell is at right angles to the 

 long axis of the cell, and it is the lower one of these cells 

 that gives rise to the embryo. The upper one forms the 

 suspensor. The embryo cell now divides once transversely. 

 /. e., in same direction of first division of egg cell. 



