Box— Vol.. I.] CANNON— A VENA FA TU A. 339 



succession, if cell-walls have been formed between them, or 

 simultaneously if the nuclei are not separated by walls 

 (figs. 18 and 19). The macrospore thus formed occupies 

 the position recently occupied by the sister cells, and extends 

 to the epidermis. No tapetum is formed. When they are 

 present the walls separating the sister cells are not so 

 strongly swollen as in Triticuni (Koernicke) and in Alofe- 

 curus (Fischer, fig. 32). In this regard the oat agrees with 

 the figures given by Fischer for Sesle^'ia. The wall next to 

 the epidermis in Avena is somewhat swollen as in Sesleria 

 (1. c. fig. 10). 



The macrospore nucleus divides into two nuclei which 

 lie, one in the micropylar end and the other at a point 

 distant about one-third the length of the embryo-sac from 

 its chalaza end. The chalaza nucleus divides at right 

 angles, and the upper nucleus divides parallel to the long 

 axis of the embryo-sac (fig. 20). One large vacuole sepa- 

 rates the two nuclear groups; and another separates the 

 chalaza nucleus from the end of the embryo-sac. Each 

 nucleus of each group divides again, the upper group at 

 right angles to the plane of the first division. The embryo- 

 sac now contains eight free nuclei (figs. 21 and 2i«). 

 During these divisions of the nuclei, the embryo-sac has 

 increased greatly in length and diameter, and has absorbed 

 the adjacent cells of the nucellus. Many large vacuoles 

 have appeared in the protoplasm of the embryo-sac; promi- 

 nent among them are the large ones surrounding the proto- 

 plasmic bridge that connects the two groups of nuclei, and 

 the vacuole in the chalaza end. 



In the micropylar end of the embryo-sac two (sister) 

 nuclei form the synergidae; and one that lies at one side of 

 the synergids, the ^^'g; while the sister nucleus of the &gg 

 becomes the upper polar nucleus (figs. 21 and 22). Three 

 of the nuclei in the chalaza end of the sac give rise to the 

 antipodal complex, and one forms the lower polar nucleus. 



The ^gg is at first spherical, but as it increases in size it 

 becomes oblong, and finally, when nearly ready for fertili- 

 zation, balloon-shaped (figs. 24 and 25). It occupies a 



