CHAPTER 9 



DEVELOPMENT AND DIFFERENTIATION 



How the Nucleus Acts: Lag How the Cytoplasm Acts: Gradients 

 Co-operation and Competition The Sequence of Events 



The development of a many-celled organism from an egg or 

 spore does not consist merely of a multiplication of cells ; it consists 

 also of the development of differences in structures and properties 

 between the cells making different tissues. The mature organism is 

 differentiated. What is this differentiation due to ? Two facts assure 

 us that it is not due to changes in the nucleus. The first is the capacity 

 of different parts of plants and animals to propagate or regenerate 

 the whole character of the organism unchanged. The second is 

 the corresponding capacity of the nucleus and its constituent 

 chromosomes to propagate themselves unchanged by mitosis. 



These two properties are opposite aspects of the principle of the 

 genetic uniformity of the parts of an individual. It is a principle to 

 which, as we have seen or shall see, there are many exceptions, due 

 mostly to chromosome, gene, and plasmagene mutations; but they 

 are rare exceptions which do not determine differentiation, and are 

 not usually even connected with it. They merely prove the rule. 



We are left, therefore, with the cytoplasm as the changing factor 

 in differentiation. The power of the cytoplasm in guiding the course 

 of events is very well shown by the alternation of gametophyte and 

 sporophyte in the mosses, ferns and higher plants. Normally the one 

 is haploid and the other diploid. But this momentous difference does 

 not decide the line which development will take in the haploid spore 

 or the diploid egg. For when the spore happens to be diploid 

 (through the failure of meiosis) or the egg happens to be haploid 

 (through the failure of fertilization) the alternation of generations 

 follows its regular sequence undisturbed. Indeed, by injury, in 

 certain mosses and ferns the gametophytic tissue can be made to 

 develop directly or "regenerate" on the sporophyte (as we saw in 

 the last chapter) without the intervention of a spore. 



It is the ordinary process of development through the alternation 



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