92 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



lar to that of the embryo sac (Fig. 59A). The enlarged nuclei 

 eoon approach the embryo sac wall, which becomes perforated at 

 such points. Gradually the pores become wider and finally the 

 entire separating wall is absorbed. The embryo sac now encroaches 

 upon these areas and soon incorporates them, coming in contact 

 with newer cells which may also meet the same fate (Fig. 595,(7) . 6 

 Even at the 4-nucleate stage, as many as 10 or more nucellar cells 

 may become included inside the embryo sac in this fashion. Their 

 nuclei divide synchronously with the sac nuclei, resulting in the 

 formation of 8 haploid and a variable number of diploid nuclei 

 (Fig. 59 D). The secondary nucleus attains varying degrees of poly- 

 ploidy, depending not only on the number of the nuclei which take 

 part in the fusion but also on their chromosome content. In the 

 mature embryo sac (Fig. 59F), the egg apparatus contains haploid 

 nuclei only; some of the antipodal cells contain haploid nuclei, 

 others diploid; most of the lateral cells (Fig. 59E) are diploid. 



The third possibility, i.e., an increase in the number of nuclei 

 caused by further divisions of the original nuclei of the sac, is rare 

 except with regard to the antipodal nuclei or cells, for which see 

 page 134. To mention some examples from recent literature, 

 in Crassula schmidtii and Umbilicus intermedins (Mauritzon, 1933) 

 it is reported that occasionally there is a fourth division in the 

 embryo sac, resulting in the formation of 16 nuclei, which organize 

 to form four synergids, two eggs, six antipodal cells, and four polar 

 nuclei. In Crepis capillaris (Gerassimova, 1933) some supernu- 

 merary egg cells were occasionally seen in addition to the other 

 and usual components of the embryo sac, but their origin could 

 not be traced and eventually they were found to degenerate and 

 disappear. In Nicotiana, Goodspeed (1947) has recently reported 

 some embryo sacs having 9 to 16 nuclei, — "obviously the result of 

 division of from one to all of the normal eight nuclei." Here 3 to 

 5 nuclei were found to take part in polar fusion. 



Special mention may be made of the development of the embryo 

 sac in Balanophora and Langsdorffia (Fagerlind, 1945a,6). In both 

 cases the micropylar megaspore functions and the three chalazal 

 mega spores degenerate at a very early stage, although their re- 



6 Harling (1946) reports that in Carludovica the nucellar cells enlarge and push 

 against the wall of the embryo sac, but in this case their contents do not actually 

 enter the sac. 



