408 



PART IV. CLASSIFICATION. 



spore is entirely filled with this tissue which constitutes the 

 prothallium. 



In the Angiosperms the development of the prothallium is more 

 complicated in that it generally takes place in two stages, the one 

 preceding, the other following, fertilisation. The nucleus of the 

 macrospore divides into two : of these the one travels to the micro- 

 pylar pole, the other to the chalazal pole of the macrospore ; each 

 nucleus then divides, and each of the four so formed divides again, 

 so that eight nuclei are formed, four at the micropylar, and four at 

 the chalazal pole of the macrospore ; one nucleus is then conveyed 



from each pole toward the 

 centre of the macrospore, where 

 the two nuclei meet and fuse 

 into one which is termed the 

 definitive nucleus of the ma- 

 crospore or embrj-o-sac. Three 

 nuclei now lie at each pole, 

 and around these aggregation 

 of protoplasm takes place, so 

 that cells are formed : those at 

 the chalazal pole soon acquire 

 a cell-wall, and are termed 

 antipodal cells : those at the 

 micropylar end do not form 

 any cell-wall ; one of them is 

 the female reproductive cell or 

 oosphere, the other two are 

 sterile (though in rare cases 

 they are fertile), and are 

 termed the syncrgidcp, the 

 three together constituting the 

 egg-apparatus. This is the 

 extent to which the develop- 

 ment of the female prothallium 

 takes place previously to fer- 

 tilisation (Fig. 242.) In most 

 Angiosperms the structure of the prothallium is completed by the 

 formation, after fertilisation has taken place, of additional cellular 

 tissue : this process is initiated by the division of the definitive 

 nucleus of the macrospore, nuclear division is repeated, cell-forma- 

 tion takes place in the manner described above for the G-ymno- 



FIG. 242. The female prothallium of An- 

 giosperms, shown in a longitudinal section of 

 the ovule, before fertilisation ( x 70) : ai outer, 

 ii inner, integument ; m micropyle ; / funicle. 

 K Maero*porangium (nucellus). E Macro- 

 spore (embryo-sac), fc Definitive nucleus of 

 the embryo-sac. The female prothallium 

 consists of the egg-apparatus at the micro- 

 pylar end of the macrospore, and of the group 

 of antipodal cells at at the chalazal end. The 

 egg-apparatus consists of two synergidse s, 

 and an oosphere e. 



