GERM CELLS 7 



Pteridophyta, are developed, and these bear the haploid gametes 

 which fuse and give the new sporophyte. The gametophyte and 

 sporophyte may be indistinguishable from one another morpho- 

 logically (as in most Rhodophyceae) save in the production of sexually 

 differentiated germ-cells by the gametophyte and of non-motile 

 spores by the sporophyte. Usually, however, the two generations 

 are sharply distinct and the less important generation (as in the 

 Bryophyta and in the flowering plants) may be parasitic on the 

 more important one. Table I classifies the main groups of animals 

 and plants in this respect (cf. Belar, 1926 h ; M. Robertson, 1929 ; 

 Naville, 1931, for Protista ; Correns, 1928, for Phanerogama ; 

 Hartmann, 1929, a and h, for lower plants ; Witschi, 1929, for 

 Metazoa) . 



(iii) Sexual Differentiation : The Gametes. Sexual differentiation 

 consists in the production by the species of two kinds of germ-cells 

 which are complementary. It is determined by the same kinds of 

 conditions as all other differentiation. But it has such special 

 importance in reproduction and in the evolution of particular genetic 

 mechanisms that it requires special consideration. 



Sexual differentiation is not found in the Flagellata, 

 or in many Phycomycetes and Chlorophyceae, and it is negligible in 

 the Rhizopoda, Gregarina (Sporozoa) and in the Basidiomycetes 

 and other Fungi. 



But in all other sexually reproducing organisms including many 

 Algae and Protozoa there is a differentiation between a small, active 

 or motile male gamete, a spermatozoon or spermatozoid, and a large, 

 inactive or non-motile female gamete, the e^g cell, usually charged 

 with nutritive material and having a larger nucleus. 



When the male gamete, or the spore where there is a haploid 

 generation, arises at meiosis, the cytoplasm of its mother cell is 

 equally divided among the four daughter cells (except in certain 

 Diptera, cf. Metz, 1926, and in the Cyperaceae, Piech, 1928). The 

 female gamete arises (except in certain Mollusca, Turbellaria and 

 oogamous Algae) by the suppression of all but one of the products of 

 meiosis. This one becomes the egg nucleus, while the other products 

 are extruded as the " polar bodies," which at once degenerate. 

 Where a short haploid generation occurs (reduced and specialised for 



