THE EVOLUTION OF STRUCTURES 157 



of turns in the spiral band of the sperm in Cycas^ the 

 large number of sperms in Microcycas, the basal haustoria 

 of the pollen tube of Ceratozamia, and other equally 

 definite features which would enable one to make a 

 fairly rehable, although impractical, key to the genera 

 based upon pollen-tube structures alone. 



As far as the male gametophyte is concerned, we 

 cannot trace any series like the reduction of the sporo- 

 phyll during the evolution of the female cone. The 

 pollen tube of Microcycas, with its numerous sperms, 

 is undoubtedly nearer the fern condition than any of the 

 rest, but the other genera seem so uniform that we have 

 not attempted to construct an evolutionary series within 

 the group. 



However, there is no doubt that these male gameto- 

 phytes are highly specialized, and that the pollen-tube 

 habit is a comparatively late development. It is prac- 

 tically certain that the male gametophytes of the 

 Paleozoic seed plants had no pollen tubes. The pollen 

 grains of these ancient forms, as far as they have been 

 observed, indicate that the male gametophyte developed 

 within the pollen grain and consequently had no green, 

 independent cells. The transition from a green, inde- 

 pendent male gametophyte protruding beyond the spore 

 to the colorless, dependent gametophyte included within 

 the spore took place in the ancient heterosporous ferns 

 which preceded the earhest seed plants. The earhest 

 pollen tubes were mere haustoria and did not carry the 

 sperms. 



EMBRYOGENY 



In the development of the embryo from the ferti- 

 lized egg some prominent features are common to all 



