THE EMBRYO 



19 



of the archegonium, which they may be seen to enter, and finally one sper- 

 matozoid coalesces with the ovum (Fig. 22). The male nucleus has been 

 seen to enter into the female nucleus, 

 and their complete fusion follows (Fig. 

 23). Thus the presence of external fluid 

 water is essential for fertilisation in 

 Ferns. Their normal life-cycle cannot 

 be completed without it. 



The immediate consequence of fer- 

 tilisation is growth and segmentation 

 of the zygote, which first secretes a cell- 

 wall. It divides first into two, by a basal 

 wall, the plane of which includes the 

 axis of the archegonium : then into 

 octants, four of which constitute an 

 epibasal heinisphere, directed towards 

 the apex of the parent thallus, giving rise to axis and leaf oi the sporeling ; 

 four form a hypobasal tier, which gives rise to the first root and a suctorial 

 organ called the foot (Fig. 24, a, b, e). These parts are soon distinguishable 



2.3- 



Horizontal section of an egg showing 

 coiled male nucleus within the female. Twelve 

 hours. (xi200.) (After Shaw.) 



Fig. 24. Young embryos of Ferns, orientated with the archegonial neck 

 downwards. The epibasal liemisphere is seen to the left, and the 

 hypobasal to the riglit. a = two-celled embryo Adianhim concinnuni 

 ( X 30 times scale). /> = similar embryo of Ptei'is serrulata ( x 30 times 

 scale). f = more advanced embryo of Adiantiim concinmiin: the 

 epibasal hemisphere has given rise to stem and leaf, and the hypo- 

 basal to root and foot. (After Atkinson.) 



by their form and structure, and are seen in their relative positions, but still 

 enclosed in the enlarged venter of the archegonium, in Fig. 25. Soon the 

 cotyledon and first root burst their way out : the former expands as the 

 first nutritive leaf, the latter buries itself in the soil (Figs. 26, 27). At first 

 the young Fern-Plant is dependent upon the prothallus that encloses it, but 

 by means of its cotyledon and its root it soon becomes self-dependent, and 

 the prothallus rots away. It is then only a matter of time and opportunity 

 for it to attain characters similar to those of the parent Fern-Plant, 



2 — 2 



