12 CELL GENETICS 



The male gamete migrates through the cytoplasm of pollen-tube 

 and embryo-sac and fuses with the egg nucleus. A second, sister, 

 generative nucleus fuses with the product of fusion of two " polar 

 nuclei " in the embryo-sac to give the " triple fusion-nucleus " 

 from which the nutritive endosperm develops, a new " triploid " 

 organism with three nuclear complements, and independent of its 

 sister embryo (Fig. 3). It has been shown that this second 

 fusion may sometimes fail in Zea. 



The female gametophyte, or embryo-sac, and the two new plants, 

 embryo and endosperm derived from it, although morphologically 

 mere organs of the parent sporophyte, are genetically three distinct 

 individuals. Since the endosperm is preponderantly maternal in 

 origin (as shown cytologically by its origin and genetically by the 

 properties it inherits) and its development is essential to the develop- 

 ment of the embryo, for which it provides nutritive material, it 

 follows that differences in seed production in reciprocal crosses 

 between species, such as are frequently observed in the angiosperms, 

 may be referred to the different relationships between embryo and 

 endosperm (cf. Thompson, 1930 ; Watkins, 1932 ; Kihara and 

 Nishiyama, 1932) as well as to the relationship between the pollen 

 and the maternal tissue which is concerned with incompatibility. 



The new individual always arises from the fusion of the two 

 germ-cells, and no others contribute normally. But it has been 

 possible to show genetically that two fertilisations may take place 

 side by side and the products join together to give a mosaic individual 

 {cf. Ch. XI). In such cases probably another cell, the second polar 

 body, functions as an auxiliary egg-cell. It is possible also that 

 two pollen grains may function in fertilising the same egg-cell, 

 both fusing with its nucleus, to give a triploid individual (Ishikawa, 

 1918, on CEnothera). 



3. HEREDITY 



(i) Genotype and Environment. Heredity is the property that 

 organisms have of reproducing their like. It must be supposed, 

 deterministically, that organisms carry something that they give to 

 their offspring, and that this " something " determines the likeness. 

 Under the simplest conditions, in the asexual reproduction of a 



