132 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



the present the origin of the embryo sac (whether mono-, bi-, or 

 tetrasporic), we shall now confine our attention to the organiza- 

 tion of the mature stage only. 



The Egg Apparatus. Typically 



the egg apparatus is composed 

 of an egg and two synergids. As 

 a rule each of the synergids is 

 notched by an indentation result- 

 ing in the formation of a promi- 

 nent hook (Fig. 85). The upper 

 part of the cell is occupied by the 

 so-called "filiform apparatus" 

 which shows a number of stria- 

 tions converging towards the 

 apex. The nucleus lies in or just 

 below the region of the hook and 

 the lower part of the cell contains 

 a large vacuole (Dahlgren, 1928a, 

 1938). In the egg, on the other 

 hand, the nucleus and most of the 

 cytoplasm lie in the lower part of 

 the cell and the vacuole in the 

 upper. Hooks and indentations 

 are usually absent, having been 

 described only in Plumbagella, 

 Ditepalanthus, Hclosis, and a few 

 members of the Ulmaceae and 

 Urticaceae (Fagerlind, 1943). 



Usually the synergids are eph- 

 emeral structures which degener- 

 ate and disappear soon after fer- 

 tilization or even before it. In 

 some cases, however, one or both 

 of them may persist for a time and 



Fig. 85. Mature embryo sac of Oeno- 

 thera nutans, showing synergids with fili- 

 form apparatus and indentations. Note 

 that nucleus of synergids lies towards 

 upper end of cell, and vacuole towards 

 lower end. (After Ishikawa, 1918.) 



show signs of considerable activ- 

 ity. In Allium unifolium and A. rotundum (Weber, 1929) this be- 

 havior is particularly pronounced, and one of the synergids begins 

 to degenerate only after the development of the embryo is well under 

 way. Nothoscordum (Stenar, 1932), Limnanthes (Fig. 86B) (Fager- 



