THE SEQUENCE OP EVENTS 



o{ a cross between them, however, some XY plants appear which 

 bear female organs. From these Winge has been able to establish 

 new hermaphrodite lines. Thus the regular progression of develop- 

 ment must be determined by genes, for the ancestral order had been 

 broken by the mixing of genes from the two stocks. The males 

 were no longer clear males: they had lost some of their distinctive 

 features. In each sub-species the genes had been balanced to give 

 the same result, but they had been balanced in different ways. How 

 this can happen is part of a much larger problem into which we 

 shall have to inquire later. 



Summing up : we may examine development and differentiation 

 in plants or in animals or in micro-organisms, in individual cells 

 or in vast aggregates; we may approach the problem by way of 

 hybridization or of transplantation. But whichever way we go about 

 it, we fmd that the centre of the control of development is the 

 nucleus whose sphere of action is the cell. The nucleus, by way 

 of the cell, controls both heredity and development. In development 

 the nucleus provides the constant basis for the regular changes in 

 the cytoplasm, changes which could not be regular unless they had 

 that constant basis. The nucleus and the cytoplasm have long 

 seemed to be the opposite poles of biological study; but their new 

 unity need no longer surprise us. Heredity now appears as the 

 repetition of the series of changes which constitutes development, 

 and the study of each is bound to require, more and more, the 

 understanding of the other. 



REFERENCES 



ANDERSSON-KOTTO, I., and GAIRDNER, A. E. 1936. The inlieritance of apospory in 

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BELAR, K. 1928. Die Cytologischen Grimdlagen der Vererbung. Berlin. 



BOUNURE, L. 1939. VOrigine des Cellules Reproductrkes. Paris. 



BOVERi, T. 1910. Die Potenzen der /l^cdrii-Blastomeren bei abgeanderten For- 

 schung. Fest. R. Hertwig (Jena), 3: 133-214. 



BRACKET, J. 1947. Nucleic acid in the cell and the embryo. Syiiip. Soc. Exp. Biol, 

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BARBER, H. N. 1941. Chromosome behaviour in Uvularia. J. Genet., 42: 223-257. 



BARBER, H. N. 1942. The pollen-grain division in the Orchidaceae. J. Genet., 

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DARLINGTON, c. D., and LA COUR, L. F. 1941. The genetics of embryo-sac develop- 

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