PERIODICITY IN DEVELOPMENT. II 



361 



antipodal cells, which, however, appear to have no special function to perform. 

 Finally, there remain over two other nuclei — the polar nuclei — which move 

 towards the centre of the megaspore, fusing sooner or later in that situation, 

 the product being known as the secondary nucleus of the embryo-sac. Which 

 of these nuclei or cells are to be considered as belonging to the prothallus it is 

 impossible to say ; this much, however, is certain, that the three cells of the 

 egg-apparatus, or, at all events, the one distinguishable by its size as the ovum, 

 represents the archegonium reduced to its simplest condition. 



After entry one of the sperm nuclei fuses with the ovum, and the product 

 develops into the embryo. The two cells adjacent to the ovum disappear. The 

 other nucleus fuses with the embryo-sac-nucleus (see p. 370), and from the pro- 



Fig, no. Pistil of Polygonum convolvulus, fii^ the funicle 

 of the ovule, supporting the body of the ovule with its integuments, 

 ie and «', and containing the nucellus, mi. The latter encloses the 

 embryo-sac, ^, in which is seen the egg-apparatus, ei^ the embryo- 

 sac nucleus, ek^ the antipodal cells, an. The ovule is enclosed 

 within the ovary,_/i£<, which is continued into the style, ^, and ends 

 in the stigma, n. On the stigma are seen pollen-grains, p, one of 

 which is giving rise to a pollen-tube, ps. x 48. From the Bonn 

 Textbook. 



Fig. III. Funkia ova/a. Upper end 

 of the embryo-sac, showing the egg- 

 apparatus. ^, before fertilization ; 8, 

 during fertilization, o, ovum ; j, syner- 

 gidae ; /, pollen-tube ; n, nucellus. x 390. 

 From the Bonn Textbook. 



duct of that fusion and from the protoplasm of the embryo-sac there arises 

 a cellular tissue — the endosperm — which becomes filled with reserve food 

 materials, and sooner or later is used up by the developing embryo. The whole 

 megasporangium now increases greatly in size and forms the seed, which when 

 mature enters on a period of rest. 



Even in the very highest plants it is thus possible to demonstrate an 

 alternation of generations. The marked reduction in the gametophyte of the 

 fern is carried in this case so far that practically nothing is left of that stage 

 except the reproductive organs themselves, and the alternation becomes in 

 consequence completely hidden. It is obvious from a consideration of these 

 facts that there will be far less opportunity for variations in the regularity of 

 the succession of generations in the higher plants than in the ferns. Since the 



