418 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



Edman, 1931; Fagerlind, 1941). It seems impossible to interpret 

 the female gametophyte of angiosperms, with all its varied modes 

 of development (several of which were unknown at the time when 

 Porsch enunciated his theory), in terms of archegonium formation. 

 As is known from our knowledge of the prothalli of pteridophytes 

 and gymnosperms, archegonia are initiated only in a cellular phase. 

 In the angiosperms, on the other hand, we are taken back to the 

 2-nucleate stage of the embryo sac as the point of origin of the 

 archegonium initials, which is pushing morphology into absurdity. 

 It seems far more likely instead that the angiosperms have long 

 passed the stage of archegonia or that they never had them at any 

 time in their fossil history. 



Coming finally to the Gnetalean theory, the name which has 

 been given to it does not imply any direct derivation of the angio- 

 sperm embryo sac from that of the Gnetales. It assumes, however, 

 that in the reduction of the prothallial tissue of the female gameto- 

 phyte, the Gnetales and the angiosperms followed a more or less 

 parallel course, leading to a complete loss of archegonia and a con- 

 dition in which all the nuclei are to be considered as potential 

 gametes. Because of the similar value attached to all the com- 

 ponents of the embryo sac, it has also been called the Gleichwertig- 

 keitstheorie or "theory of equivalence." No single botanist can 

 be credited with its authorship, for it seems to have developed 

 slowly as the result of certain opinions expressed from time to 

 time by Hofmeister, Strasburger, and others. It has found sup- 

 port during recent years from further elucidations of the morphol- 

 ogy and embryology of the Gnetales in general and of the genus 

 Gnetum in particular (Thompson, 1916; Pearson and Thomson 

 1918; Fagerlind, 1941, 1946). 



Before entering into a comparison of the embryo sac of Gnetum 

 with that of the angiosperms, it may be well to recall the main 

 facts in the development and organization of both. 



Taking the angiosperms first : 



1. The embryo sac may originate from 1, 2, or all 4 megaspore 

 nuclei. 



2. Only a few nuclear divisions occur after megasporogenesis, 

 and there seems to be a tendency towards further reduction in 

 this number. 



