116 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



division proceeds normally (Fig. 73/), but of the 8 nuclei now 

 formed the basal nucleus remains undivided (Fig. 73 Jv) so that 

 the mature embryo sac shows only 15 nuclei (Fig. 73L) which be- 

 come organized to form a three-celled egg apparatus, two polar 

 nuclei, and 10 antipodals (Fig. 73M). 



Fig. 74. Development of embryo sac in Tanacetum vulgare. A, mega spore mother 

 cell. B,C, end of first and second meiotic division, respectively. D, megaspore 

 nuclei. E, same, older stage, showing vacuolation. F-H, first postmeiotic divi- 

 sion; two megaspore nuclei dividing in F, three in G, and all four in H. I, J, mature 

 embryo sacs with varying number of nuclei. (After Fagerlind, 1941.) 



In Maianthemum bifolium (Stenar, 1934) both the reduction divi- 

 sions are accompanied by the formation of cell plates (Fig. 75 A-C). 

 They soon become absorbed, however, resulting in a common tetra- 

 nucleate cell (Fig. 75D). The four megaspore nuclei take up a 

 1+3 arrangement, so that the next stage shows 2 nuclei at the 

 micropylar pole and 6 at the chalazal pole (Fig. 75E-F). There is 

 one more division, resulting in 16 nuclei (Fig. 75G). These organize 

 into an egg apparatus, two polar nuclei, and 11 antipodal cells. 



