DEVELOPMENT OF GAMETES 



349 



elongated and is surrounded by a little cytoplasm, so 

 that it forms a little naked cell within the pollen grain 



male 



m.r. 



nuc. ill 



ant. 



B 



FIG. 59. The ovule and fertilisation. A, cross-section of a single 

 free carpel containing two rows of ovules ; m.r., midrib of car- 

 pellary leaf; pi., placenta (thickened margin of carpellary leaf) 

 bearing row of ovules. B, ripe ovule (megasporangium) in 

 longitudinal section; v.b., vascular bundle; nuc., nucellus (body 

 of ovule = wall of megasporangium) ; o.c., outer coat; i.e., inner 

 coat ; micr., micropyle. The embryo sac (megaspore) contains 

 in the centre the secondary nucleus (sec.n.) ; at the micropylar 

 end the two synergidse (syn.) and the egg, and at the opposite 

 end the three antipodal cells (ant.). C, development of embryo 

 sac ; a-d, division of primary nucleus to form eight nuclei, four 

 at each end ; e., formation of egg apparatus, group of antipodal 

 cells and secondary nucleus. D, embryo sac at the time of 

 fertilisation; p.t., pollen tube, which has entered the upper end 

 of the sac and evacuated the two male gametes, one of which 

 is seen in contact with the nucleus of the egg (female gamete) 

 and the other with the two polar nuclei, which in this case have 

 not yet fused to form the secondary nucleus of the sac. Contact 

 will be followed by fusion, in the former case giving the zygote, 

 in the latter the mother nucleus of the endosperm. 



cell (Fig. 58, C). This independent cell later divides 

 to form the two male gametes (E). Thus the male 



