When a sperm-cell meets an egg-cell it penetrates the 

 outer coating and enters its substance. The tail is absorb- 

 ed and the head or nuclear part moves toward the egg- 

 nucleus which at the sam^e time moves to meet it. The two 

 nuclei merge into one. but the chromosomes of each retain 

 their identity. This mingling of the male and female nuclei 

 constitutes fertilization and the initiation of cell-division of 

 cleavage of the egg-cell. The fertilized egg-cell by repeated 

 divisions forms the embryo. 



(b) — Fertilization in Seed Plants 



The nucleus of the pollen grain divides into two — 

 the generative and the tube nuclei — about the time of shed- 

 ding of the pollen. The generative cell again divides into 

 two male nuclei or cells. After the pollen grain reaches the 

 stigma the pollen tube is formed which penetrares the style 

 to the micropyle of the ovule in the ovary, thence through 

 the nucellus to the embryo sac. The two male cells are 

 then discharged from the end of the tube on reaching the 

 egg-cell, when fertilization with subsequent development of 

 the embryo is effected; the other fuses with the endosperm- 

 or fusion-nucleus near the centre of the ,embryo-sac and 

 forms the endosperm surrounding the embryo (Fig. 4). 



(c) — Fertilization in Animals 



Fertilization in animals, as in plants, consists in the 

 fusion of a male cell with rn egg-cell. This fusion takes 

 place among the higher animals within the body of the 

 female; but in frogs and fishes the union occurs after the 

 eggs are laid. 



(d) — Development of the Embryo in Seed Plants 



(a) In Shepherd's Purse as a type of the Dicotyledon- 

 ous plants. By several transverse divisions the fertilized egg- 

 cell becomes a thread-like body called the pro-embryo. The 

 last cell divides into octants, the four terminal cells forming 

 the stem and cotyledons, the four basal cells the hypocotyl. 

 By periclinal walls the dermatogen (or primitive epidermis) 

 layer is cut off, while the periblem (or primary cprtex) and 

 the plerome (or primary stele) are soon differentiated from 

 the interior tissues by many divisions (Fig. 5). 



(b) In Arrow-head as a type of the Monocotyledonous 

 plants. By two transverse divisions a three-celled pro- 

 embryo is formed, the terminal cell forming the cotyledon. 



41 



