12 



INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



in those days being that it arose by the simple enlargement of a cell 

 of the nucellus. 



Further knowledge of the development and organization of the 

 embryo sac became available as the result of a concentrated attack 

 on the problem made by several botanists during the years 1877 to 

 1881. To be named specially in this connection are Warming, 



D E F G H 



Fig. 10. Development of embryo sac in Polygonum divaricatum. A, megaspore 

 mother cell separated from nucellar epidermis by primary wall cell. B, dyad cells 

 in division. C, tetrad of megaspores with wall cells above. D, functioning mega- 

 spore. E-G, embryo sacs, showing two, four, and eight nuclei. H , l.s. ovule, show- 

 ing mature embryo sac. (After Strasburger, 1879.) 



Vesque, Strasburger, Fischer, Ward, Jonsson, Treub and Mellink, 

 and Guignard. Strasburger (1879) demonstrated that at first one of 

 the nucellar cells becomes differentiated as the megaspore mother 

 cell (Fig. 10 A) and goes through two divisions to give rise to a row 

 of four cells (Fig. 10B,C). Of these, the three micropylar cells soon 

 degenerate and the chalazal alone enlarges and functions (Fig. 10D). 

 The nucleus of this cell (the functioning megaspore) divides thrice 

 to give rise to two groups of four nuclei, one at the micropylar end 

 and the other at the chalazal end of the cell (Fig. 10E-G). From 



