REPRODUCTION 441 



detached from the parent sporophyte and disseminated in 

 various ways. 



In the Angiosperms the formation of the seed is in the 

 main similar to the process described, hut it has certain 

 peculiar features. The embryo-sac or megaspore has the 

 same structure as in the Gymnosperms and remains en- 

 closed in the sporangium or ovule. The development of 

 the prothallium is different. The megaspore has a single 



P.I. 



FIG. 178. OVULE OF Pinus, 



SHOWING THE PROTHALLIUM; 



end, is THE MEGASPORE mac; 

 arch, archegonia. 



FIG. 179. OVULE OF AN ANGIO- 

 SPERM, SHOWING THE MEGASPORE ; 

 mac, WITH ITS PROTHALLIUM; 



005, OOSPHERE. 



nucleus as in other cases. When germination begins this 

 divides into two, one of which travels to each end of the 

 ovoid spore. Each of these gives rise by two successive 

 divisions to a group of four nuclei, and a single nucleus from 

 each group returns to the centre of the cell, where the two 

 fuse together. These are often termed the polar nuclei. 

 At this stage the prothallium ceases to undergo any change 

 (fig. 179) ; it consists of a group of three nuclei at the apex, 

 known as the egg apparatus ; another group at the base, 



