HOW THE NUCLEUS ACTS 



SO if deprived of its nucleus in the rhizoid, provided that no stem 

 w^as taken away with the hat. In other words there is a hat-forming 

 substance near the hat. If the hat is cut away with a large part of 

 the stem, it can still be formed again provided that the nucleus 

 remains present. But the regeneration then takes time — time for the 

 nucleus to make some more hat substance. 



This argument is clinched by species transplants. If the hat and 

 stem of one species is grafted onto the rhizoid and nucleus of another 

 the existing hat is unaffected. But, as we have already had occasion 

 to observe, any new hat is modified in its development, taking more 

 and more the form laid down by the nucleus as the length of its 

 association with that nucleus increases. 



Thus the nucleus is producing the materials, the proteins, which 

 eventually decide the character of the cell; but time is needed for 

 these proteins to accumulate and to become effective by replacing 

 those produced by the previous nucleus. 



The lag before a gene becomes effective is important in the 

 ordinary life of an organism. There are some genes which become 

 visibly effective within a single cell generation. In the pollen grains, 

 following segregation of waxy from non-waxy at meiosis in a 

 heterozygous maize, the two types are distinguished before the 

 pollen is ripe by the presence or absence of starch. In Paramecium, 

 on the other hand, 36 or more generations may elapse before a 

 gene takes effect. Correspondingly, in the multicellular echinoderms, 

 hybrids have been found to cleave at the maternal rate until the 

 gastrula stage is reached, when the paternal genes make themselves 

 felt. Their effects may then appear as a breakdown of development 

 when the cross has been too wide, that is, when the hybrid genes 

 act too dissimilarly from the maternal ones which have laid down 

 the character of the cytoplasm. 



Most of the characteristic genes of mendelian experiments, as we 

 have seen, act later in development; not indeed until the individual 

 is approaching maturity. Those which act more quickly act too 

 strongly : they are lethal. A few act more slowly and do not manifest 

 themselves until the next generation. The best known example is 

 in the determination of the direction of coiling of the snail's shell 

 which follows the mother's genotype whatever the character of the 

 father (Fig. 45). 



191 



