420 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



after the entry of the pollen tube. Its cells are multinucleate and 

 nuclear fusions are common, but no male nucleus has been observed 

 to take part. 



If we now compare these two sets of observations, four points 

 seem worthy of note : 



1. In both cases there is a variation in the number of megaspore 

 nuclei which take part in the development of the gametophyte. 

 It may be stated parenthetically that no other genus among the 

 gymnosperms resembles the angiosperms in this respect. 



2. There is a tendency toward reduction in the number of nuclei 

 of the embryo sac, but this is far more pronounced in the angio- 

 sperms. 



3. Archegonia are completely suppressed in both cases. 



4. There is a tendency toward a postponement in the formation 

 of the storage tissue until after fertilization, and its development is 

 preceded by nuclear fusions. 



In inviting attention to these and to certain other similarities in 

 the vegetative anatomy of the two groups, Thompson (1916) says: 

 "In regard to the angiospermic relationship almost every structure 

 described [in Gnetum]. . .shows some approach to the angiospermic 

 condition and. . .some structures show conditions almost completely 

 angiospermic. . . Such a body of evidence can scarcely be ignored 

 or put aside as the result of parallel development.. . . Accordingly 

 the sum of the evidence from all sides seems to lead to the conclu- 

 sion that angiosperms are phyletically related to Gnetales. This 

 does not mean that any modern member of the Gnetales represents 

 the type from which angiosperms were derived but that the an- 

 cestors of angiosperms were not far removed from the genus 

 Gnetum." 



Fagerlind (1941) also expresses himself in favor of "a more or 

 less intimate genetical connection between the ancestral types of 

 angiosperms and gymnosperms" and draws the following con- 

 clusions : 



1. The polar nuclei of the angiosperms are the last remnants of 

 the free nuclei seen in the female gametophyte of Gnetum and in 

 the earlier stages of development of the gametophytes of other 

 gymnosperms. 



2. The central vacuole of the angiosperm embryo sac is homolo- 



