EMBRYOGENESIS IN G YMNOSPERMS 179 



with the upper and lower regions of the zygote. Division of nuclei of the 

 lower group is always accompanied by division in the upper group but 

 the converse is not always found. From this it could be inferred either 

 that there is a gradient of a stimulatory substance from the archegonium 



Fig. 38. Fertilisation in Dioon edule 



Reconstruction of an ovule, as seen in longitudinal section, with its prothallus and 

 two of the archcgonia at the time of fertilisation. Motile spermatozoids are dis- 

 charged from the ends of the pollen-tubes into the archegonial cavity. A sperm has 

 entered the egg on the left ; the egg on the right still shows the ventral canal nucleus ; 

 two sperms, immediately above, are ready to enter this egg (after Chamberlain). 



base to the neck, or that there is a gradient of inhibitory substance in 

 the opposite direction. During the earlier phase of zygotic development 

 many more nuclei are to be observed at the upper end. The extent of 

 free nuclear formation varies in different genera and species. In the 

 large ovum of Dioon edule, which may be as much as 5 mm long, some 

 1000 nuclei are present (the corresponding theoretical number would 

 be 1024); in Zaimafloridaiia, with a 3 mm ovum, 256 nuclei have been 



