176 EMBRYOGENESIS IN PLANTS 



Stimulus from the pollen or pollen tube being involved should, perhaps, 

 also be considered. In Cycas, for example, the pollen has already been 

 received while the female gametophyte is still in the free nuclear stage. 

 This late differentiation of the female gametophyte is very general in 

 gymnosperms. In some genera, e.g. Microcycas the archegonium 

 initials may occur round the sides and even near the base of the 

 prothallus, but only those at the micropylar end become fully developed 

 and functional (Reynolds, 1924). The ventral cell of the developing 

 archegonium, almost all of which will become the ovum on maturation, 

 enlarges rapidly, and becomes highly vacuolated. This would indicate 

 the abundant presence of substances which increase osmotic pressure. 

 A rapid inward movement of protein-forming substances then follows 

 and a protoplasmic reticulum, with a centripetal gradient of small to 

 large vacuoles, is formed. Meanwhile, the nucleus, which has also 

 enlarged, divides, the daughter nuclei becoming the ovum nucleus and 

 the ventral canal nucleus. The latter soon becomes disorganised. It 

 has, however, occasionally been observed to enlarge like the egg nucleus 

 and it may, in some instances, function as a male gamete, fusing with 

 the egg nucleus (Sedgwick, 1924; Chamberlain, 1935). The archegonial 

 neck in gymnosperms is very small, and, by comparison with that in 

 bryophytes would be regarded by morphologists as being very much 

 reduced. 



As the ovum enlarges, it becomes surrounded by a tapetal-like layer 

 of cells known as the archegonial jacket. As this jacket becomes more 

 highly differentiated, the egg-membrane in contact with it becomes 

 progressively more thick and tough, so resistant indeed that it remains 

 firmly attached to the suspensor in maturing seeds. During its early 

 development the egg-membrane in Cycas has numerous large pits, each 

 covered by a thin membrane. These tend to be thrust by the turgid 

 protoplasm of the egg into the adjacent jacket cells as haustorium-like 

 processes. As the maturing egg becomes more turgid, these processes 

 are protruded still further into the jacket cells, the separating lamellae 

 and walls are broken down, and direct contact is established between 

 the egg protoplasm and that in the jacket cells. At this stage the cells 

 of the prothallus are densely packed with starch, protein and probably 

 other reserve materials. These are the materials which are being 

 actively drawn upon by the ovum through its peripheral haustoria. 

 According to Chamberlain : 'The turgidity of the female gametophyte 

 (in the cycads), and later, the turgidity of the central cell, and, still 

 later, the egg cell, is extreme,' and he adds that if small cuts are made 

 in the prothallus near the egg membrane, 'the liquid contents of the 

 egg will spurt out, sometimes to a distance of 20 cm.' This is indeed a 

 remarkable physiological situation, quite unmatched in any of the 



