434 



REPRODUCTION. 



mi 



tc 



sac. During the subsequent development of the ovule the 

 embryonal sac continues to increase in size, often irregular^, 



and displaces or obliterates by absorp- 

 tion man}' of the cells around it. 



1127. At an early period in the de- 

 velopment of the embryonal sac it is 

 completely filled with protoplasm con- 

 taining a cell-nucleus. This nucleus di- 

 vides, and the two new nuclei are soon 

 found at opposite ends of the sac, where 

 each divides into four nuclei. Between 

 the two groups of four nuclei there may 

 be a vacuole of considerable size. 



The next stage is marked by the pas- 

 sage of a nucleus from each extremity of 

 the embryonal sac towards its centre, 

 where they become united to form a see- 

 ondarv nucleus. 



t/ 



1128. The nuclei at the lower end of 

 the sac become surrounded with other 



protoplasmic matter, and later by cell-walls ; they then consti- 



201 



tute what have been termed the antipodal cells. At the upper 

 end of the sac, also, the three nuclei become surrounded by 



FIG. 199. Longitudinal section of the orthotropous ovule of Polygonum divaricatum. 

 fu, funiculus; te, the two integuments; nil, the nucellus, whose summit is prolonged 

 towards the micropyle, mi ; se, the embryonal sac. (Strasburger.) 



FIG. 200. Polygonum divaricatum. Summit of the ovule with the apex of the em- 

 bryo sac, and the complete embryonal apparatus, e, the ob'spore; s, one of the syner- 

 gidae, the other being hidden from view. (Strasburger.) 



FIG. 201. Polygonum divaricatum. Summit of the ovule, showing the encroachment 

 of the embryo sac upon the adjoining cells. (Strasburger.) 



