FILICALES 345 



of external fluid water is essential for fertilisation in Ferns. Their 

 normal life-cycle cannot be completed without it. ^ 



FIG. 286. 



Fertilisation in Onoclea sensibilis : the arrows indicate direction to the growing 

 point. A a vertical section through an archegonium probably within ten minutes 

 after entrance of the first spermatozoid. ( x 500. ) B vertical section of the venter 

 of an archegonium, containing spermatozoids, and the collapsed egg with a sperma- 

 tozoid within the nucleus. Thirty minutes. ( x 1200.) (After Shaw.) 



The immediate consequence of fertilisation is growth and segmenta- 

 tion of the zygote, which first secretes a cell-wall. It divides into 



FIG. 287. 



Horizontal section of an egg, showing coiled male nucleus within the female. Twelve 

 hours. ( x 1 200.) (After Shaw.) 



octants, four of which constitute an epibasal hemisphere, directed 

 towards the apex of the parent thallus, giving rise to axis and leaf of 



