130 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



into the apex of the tube, undergoes two divisions to form 4 nuclei, 

 which lie in two pairs, one at each end of a large vacuole (Fig. 

 SSG-H). The next division gives rise to 8 nuclei, of which the 

 upper 4 form the egg apparatus and the upper polar nucleus, and 

 the lower 4 give rise to the three antipodal cells and the lower 

 polar nucleus (Fig. 83 1-J). The lowest antipodal cell connects the 

 vesicular outgrowth with the body of the old megaspore mother 

 cell in which the three degenerated megaspore nuclei are sometimes 

 still distinguishable. The nuclei of the antipodal cells frequently 

 undergo a few divisions but the daughter nuclei fuse once again 

 to form a single lobed nucleus. 



Chrysanthemum cinerariaefolium. Martinoli (1939) has dis- 

 covered a peculiar mode of development in this plant. The em- 

 bryo sac is tetrasporic and the megaspore nuclei take up a 1+2 + 1 

 arrangement so that there is 1 nucleus at each pole and 2 nuclei 

 lie in the middle (Fig. 84A-C). The two central nuclei become 

 separated from the terminal nuclei by vacuoles and may either 

 fuse to form a single diploid nucleus (Fig. 84#) or may merely re- 

 main close to one another without undergoing any fusion (Fig. 

 84D). The subsequent development differs, depending on which 

 of the two conditions is present. 



In the first case the next division gives rise to 6 nuclei (a haploid 

 pair at either end and a diploid pair in the center) (Fig. 847) which 

 divide again to form three groups of 4 nuclei each (Fig. 84/). The 

 micropylar quartet now produces the egg apparatus and upper 

 polar nucleus, all haploid. The chalazal quartet gives rise to four 

 antipodal cells, also haploid. The central quartet is composed of 

 diploid nuclei; one of these functions as the lower polar nucleus and 

 the remaining 3 organize as additional antipodal cells (Fig. 84iv). 

 Sometimes less than 12 nuclei are formed (10 or 7), either because 

 of a failure of some divisions at the chalazal end or because the 

 central diploid nucleus of the 3 -nucleate stage undergoes only one 

 division instead of two. 



In the second of the two previously mentioned alternatives, i.e., 

 when the two central megaspore nuclei do not fuse but only lie in 

 contact with each other, neither undergoes any further divisions 

 and both function directly as the polar nuclei. Meanwhile the 

 micropylar megaspore nucleus divides twice, to give rise to the 

 micropylar quartet, but there is no regularity in the behavior of 



