93 



further growth makes it appear above the middle (fig. 59 

 PI. VIII, fig. 61 PI. XI), while finally, when all the nucellar 

 tissue has been destroyed by the growth of the embryosac- 

 content it can't be noticed any more. 



Up till now there was nothing inside of the embryosac but 

 free nuclei, after this a formation of cells takes place at the 

 chalazal end, which soon takes the definite shape of a kind 

 of prothallium (fig. 29 PI. IV). This prothallium as I will call 

 it is undoubtedly present, at least in many cases, before fer- 

 tilisation has taken place, that is before a pollentube has reached 

 the embryosac. This fact can easily be established by setting 

 the embryosac free by means of a pair of needles and turning 

 it around and around. If pollentubes have reached the em- 

 bryosac they can thus easily be seen. I am not able to state 

 definitely wether it is formed without pollination that is without 

 the stimulus of a pollentube growing into the nucellar tissue, 

 but it is certain that it is formed before the pollentube has 

 reached the embryosac, it consequently is no product of fer- 

 tilisation as little as the formation of an ovule in flowers, 

 pollinated at a time when no trace of an ovule existed (some 

 Cupuliferae etc.), can be considered the product of fertilisation. 



It will be seen, later on, that this prothallium plays a role 

 entirely independant from the other parts of the embryosac 

 and has therefore full right to be considered as a special for- 

 mation, I might almost say as an organism. 



An embryosac of Gnetum Gnemon L. consequently consists 

 at this moment of a sac, constricted at some point below the 

 middle, the lower end up to the constriction containing a 

 prothallium, the part above it containing a large number of 

 free nuclei. In this stage the embryosac can be fertilised, no 

 further changes occur. 



As the upper part only plays a role in the process of repro- 

 duction I will call that the fertil half of the embryosac, the 

 chalazal end the steril half. 



