180 STRUCTURAL BOTANY 



very large number of nuclei. After a time a sort of 

 papilla or beak is formed (Fig. 76, A) on one side. A 

 remarkable change now takes place in the contents. 

 A portion of the protoplasm wanders back into the 

 filament, carrying with it all the nuclei except one, so 

 that the oogonium becomes a uninucleate cell. It 

 is now cut off by a transverse wall from the filament. 

 The contents rearrange themselves so that the portion 

 towards the papilla becomes clear and free from 

 chloroplasts, forming the receptive spot. The now solitary 

 nucleus (which has grown rather larger than before) 

 lies towards the middle of the cell, connected by a 

 strand of protoplasm with the clear receptive spot. 

 The wall of the oogonium opens at the papilla, and at 

 the same moment a portion of the colourless protoplasm 

 is expelled. It is probably due to the pressure of the 

 protoplasmic mass that the wall is opened; sometimes 

 the extruded part of the contents remains in connection 

 with the protoplasm within the oogonium. From this 

 point onwards we may speak of the contents of the 

 oogonium as the ovum. 



Fertilisation now takes place. The minute sperm- 

 atozoids swarm in at the oogonial aperture, but do not 

 necessarily select the particular oogonium near which 

 they were formed. In Fig. 76, B, for example, an 

 antheridium is shown which has not yet discharged 

 its spermatozoids, although the adjoining oogonium 

 is actually being fertilised. Numerous spermatozoids 

 may enter an oogonium, but only one fuses with the 

 ovum, which it penetrates at the receptive spot where 

 the protoplasm is clear. The nucleus of the sperm- 

 atozoid has been traced through the protoplasm of the 

 ovum, until it reaches the sole remaining nucleus, with 



