10 CELL GENETICS 



in hybrids with irregular meiosis, with a disorganised or incomplete 

 nucleus or with a multiple nucleus. Spermatozoa with either too 

 little (" oligopyrene " or " apyrene "), or too much, nucleus may 

 develop the external apparatus of a sperm, or even function to give 

 progeny. Kuhn (1929 6) found that sperms lacking certain chromo- 

 somes (the X and Y) functioned in fertilisation in Drosophila, 



When pollen grains have too little nucleus, on account of their 

 more complicated vegetative life, they do not produce functional 

 gametes or even develop as far as their vegetative division {cf. 

 Ch. VIII). When grains with too much nucleus function, they are 

 of great importance in connection with the origin of polyploids and 

 unbalanced forms (Chs. V, VIII, and X). 



The fusion of the nuclei in fertilisation usually takes place directly, 

 but the process shows great variation. The two bodies of 

 chromosomes may first unite at the mitosis which begins imme- 

 diately before the nuclei meet {e.g., Ascaris). 



Two special types of fertilisation require consideration. In some 

 fungi, fusion of nuclei does not immediately follow the fusion of 

 their cells. In the Basidiomycetes the haploid spore develops into 

 a mycelium containing haploid nuclei. The mycelium is at first 

 multinucleate, but later cell-walls are formed so that each cell 

 contains one haploid nucleus ; this is the haplophase. Pairs of 

 cells then fuse, derived from the same plant or mycelium in " homo- 

 thaUic " species or from different mycelia in " heterothallic " 

 species. But their nuclei do not fuse ; they exist and divide at 

 each cell division, cohabiting throughout the diplophase. Eventually 

 these " conjugate nuclei " fuse in the reproductive organ, the 

 basidium, and the next division is meiosis which gives the four 

 haploid spores. Thus, while the cells are diploid in the diplophase, 

 the nuclei are haploid. Similar conditions are found in the Asco- 

 mycetes (cf. Winge, 1935, on Saccharomyces). The significance of 

 this remarkable behaviour will be considered later (Fig. 2). 



In the flowering plants one kind of gametophyte, the pollen grain 

 or microspore, produces the male gamete, its " generative nucleus," 

 after undergoing two haploid mitoses (cf. Geitler, 1935), while the 

 other, the embryo-sac cell or megaspore, produces the female 

 gamete, usually after three mitoses, together with seven other 



