ix FILICINE& LEPTOSPORANGIATJE 321 



spot, Shaw states that the egg then collapses, and suggests that 

 this prevents the penetration of more than one spermatozoid. 

 Mottier ((3) p. 139) expresses some doubt whether the 

 collapsed appearance of the egg, usually found in microtome 

 sections, is really normal. 



The spermatozoid soon penetrates into the nucleus of the 

 egg, where for some time it remains with little change of form. 

 Presumably the cilia and the cytoplasmic part of the sperma- 

 tozoid remain in the egg-cytoplasm as they do in Cycas and 

 Zamia (Ikeno (i), Webber (i)). 



The body of the spermatozoid, after it penetrates the egg- 

 nucleus, gradually loses its homogeneous appearance, and the 

 nuclear reticulum becomes more and more apparent. The 

 spiral form becomes less evident, and the nucleus passes through 

 much the same changes, except in reverse order, that are seen 

 in its development from the nucleus of the sperm-cell. Finally 

 the reticulum of the male nucleus becomes indistinguishable 

 from that of the egg-nucleus, and the fusion is complete. Dur- 

 ing this fusion the egg nucleus retains its original form. 

 The process of fusion is slow. In one instance, sixty 

 hours after fertilisation, the sperm-nucleus was clearly recog- 

 nisable. 



As soon as the egg is fertilised it develops a membrane, 

 and soon after undergoes its first segmentation. The inner 

 walls of the neck cells almost immediately turn dark brown, 

 and the cells of the ventral part begin to divide actively and 

 form the calyptra, which here, as in the Bryophytes, is formed 

 from the venter alone, and is tipped with the remains of the 

 neck cells. 



The position of the archegonium depends largely upon the 

 light. If both sides of the prothallium are about equally 

 illuminated, archegonia will develop from both sides. As soon 

 as an archegonium is fertilised, no new ones form, but it fre- 

 quently happens that a very large number prove abortive before 

 finally fertilisation is effected. 



The Embryo 



The first division wall in all Polypodiacese yet investigated 

 is vertical and nearly coincident with the axis of the arche- 

 gonium. This basal wall (Fig. 178, A) at once divides the 



21 



