103 



faced how can the difference of the two halves of the macros- 

 poreal content be explained*? Nothing remains but to consider 

 the whole content of the embryosac of Gn. Gnemon as the 

 homologon of the Welwitschia prothallium. 



The upper part of the prothallium must have been reduced 

 to its bare necessities viz. the nuclei, while the lower part 

 remained as it was in Welwitschia. Gnetum Gnemon conse- 

 quently is an intermediate form between Welwitschia and the 

 Gneta described by Karsten. 



The difference in the two halves of the embryosac of Gnetum 

 Gnemon is consequently due to a reduction in the upper part 

 of the prothallium , a reduction which in the other Gneta has 

 allready extended over the whole of the prothallium. 



The subsequent growth of the lower part of the prothallium 

 and its serving as a foodsupply to the embryo's originated in 

 the upper half must have been secondarily acquired. The for- 

 mation of some „endospermic" cells in the fertil part after ferti- 

 lisation has taken place is simply a retarded prothalliumformation. 



The occasional formation on the lower part of the prothal- 

 lium of archegonium-like bodies must be considered as a case of 

 atavism reaching backwards outside of the limits of the genus. 

 Concluding and recapitulating I think the Gnetaceae must be 

 considered to be entirely independent in their origin of the An- 

 giosperms. No Gnetacea has ever been an Angiospermic-ancestor. 



If the few forms of the Gnetaceae now existing had become 

 extinct nothing would have made us conclude that such a 

 groupe had ever existed. 



While in the Angiospermic line of descent the archegonium 

 has been reduced to a simple cell, the reduction in the Gnetum- 

 line has been pushed to its utmost limit viz. to a single nucleus. 



IS A COMPARISON BETWEEN THE CONTENTS OF THE ANGIOSPEUMIC- 

 EMBRYOSAC AND THOSE OE THE GNETUM-EMBRYOSAC POSSIBLE? 



Before going into this question it will be well to see what 

 difficulties we meet in the interpretation of the contents of 

 the Angiospermic-embryosac. 



