102 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



but is actually bisporic as a result of an early dissolution of the 

 cell walls laid down after the second meiotic division. 



In 1907, Pace published an interesting paper on the develop- 

 ment of the embryo sac in four species of Cypripedium. According 



G H 



Fig. 66. Development of embryo sac in Convallaria majalis. A, l.s. nucellus, show- 

 ing megaspore mother cell. B, dyad stage. C, T-shaped tetrad. D, wall sepa- 

 rating the two megaspores of each dyad cell has disappeared. E, the two dyad 

 cells, each binucleate. F, upper dyad cell degenerating; lower enlarging. G, 

 four-nucleate embryo sac formed from lower dyad cell. H, same, more advanced 

 stage. /, mature embryo sac. (After Stenar, 1941.) 



