DEVELOPMENT AND UI FFERENTI ATION 



the pollen grain, much as the excessive mitosis of a tumour may 

 kill an animal. 



Pollen grains provide us, both internally and, as we saw earlier, 

 externally, with crucial tests of developmental principles simply on 

 account of their unique property of being independent organisms 

 limited as a rule to two or three cells. The different sequences of 

 their development arise from the reactions of different types of 



Fig. 47. — The four pollen grains of a tetrad in Scilla sibirica still exceptionally 

 adhering attcr the first mitosis. As in Ascaris (Fig. 48), a radial spindle has given normal 

 differentiation in three cells; an exceptional tangential spindle has given no dif- 

 ferentiation in the fourth. The concentration of gcnerative-nucleus-forming 

 substances, represented by stippling, is evidently central in the pollen mother cell 

 (La Cour, original). 



nuclei, symmetrically or asymmetrically placed with respect to the 

 spatially differentiated cytoplasm of a single cell. The immense range 

 of types of embryo-sacs in flowering plants depends on the same 

 principles operating in more elaborate settings, showing a range 

 of behaviour depending, no doubt, on varying gradients in the 

 distribution of nucleic acid in the mother cell (Fig. 50). 



Similar to the differentiation in space in the blood and pollen 

 is that in time, whereby the series of mitoses in the development of 

 a plant or animal is interrupted by meiosis, as a result, perhaps, of 

 flooding of the nucleus with nucleic acid and the charging of the 



196 



