THE GYMXOSPERMAE : CYCADALES, ETC. 729 



this osmotic pressure and discharges its fluid contents with the antherozoids 

 into the archegonial chamber. The strong fluid draws water from the 

 archegonial neck cells and from the oosphere, in which vacuoles appear. 

 When an antherozoid touches a neck cell it is sucked violently into the 



Cilia 

 Blepharoplast 



I- Nucleus 



Cytoplasm 



Microspore wall 



Fig. 733. — Ceratozamia mexicana. Portion of a microspore 

 showing the antheridial body cell dividing to form two 

 antherozoids with very large nuclei. 



oosphere, the cytoplasm and blepharoplast are stripped oft", and the naked 

 nucleus approaches and unites with the oosphere nucleus. 



Embryogeny 



As in Pinus, and apparently in Angiosperms, the chromatin of the 

 two nuclei does not immediately fuse, and when division begins there 

 are two spindles formed, which merge into one. Only the monoploid 

 number of chromosomes can be counted, and the explanation is probably 

 the close pairing of male and female chromosomes which makes them 

 indistinguishable. 



The earlv divisions of the oospore nuclei are free and simultaneous and 

 two to three hundred nuclei are scattered throughout the oospore. The 



24 A 



