io THE COTTON PLANT IN EGYPT CHAP. 



connection with the chromosome groups by fine fibres 

 (Fig. 11), which become the spindle fibres. The pseudo- 

 tetrads arc then halved into groups of pairs, the inter- 

 chromosome fibre disappears in the cytoplasm, and the 

 two daughter nuclei rapidly divide again (Figs. 14, 15) in 

 the same way, never breaking the thread-rings, and thus 

 distribute one minute chromosome from each pseudo-tetrad 

 to each of the four microspores. 



The prominence of the non-chromatic linin structures in 

 this remarkable division has enabled these changes to be 

 examined thoroughly. The main features are summarised 

 above, and the possibility of providing a physical explana- 

 tion for nuclear division on these lines makes the observa- 

 tion one of general interest. The same thread-rings are 

 found in the somatic nuclei (Figs. 16, 17, 18), where the 

 chromosomes number forty, and the same scheme is 

 followed. 28 



The spores. The microspores, or pollen -grains, are 

 thus formed in groups of four. Each member of the group 

 enlarges (Fig. 19), develops spiny sculpturings on its 

 outer wall, colouring matter golden in the case of 

 Egyptian cotton and finally floats free in the liquefied 

 residuum of the tapetum. Some three days before the 

 flower opens, the single nucleus divides into a moruloid 

 vegetative nucleus and a smaller ellipsoid generative 

 nucleus (Fig. 20). The latter again divides into two, the 

 male gametes, and the pollen grain is then ready to 

 fertilise the ovary. 



The megaspores are also formed in groups of four, but 

 the three nearest the base of the ovule abort, and only the 

 fourth member becomes a megaspore. The usual nuclear 

 changes then take place : the three antipodals abort before 

 the flower opens, while the two polar nuclei have met in 

 the centre of the spore, though without fusion, and two 

 typical synergids support the egg-cell or female gamete. 



Fertilisation. We shall have ample occasion later to 



