Ui;VELOPMENT AND Dli^I HREN TI ATIO N 



chromosomes before they have reproduced themselves. Diftbrentia- 

 tioii in space as well as in time is responsible for the fragmentation 

 of the chromosomes in particular embryonic cells in Ascaris. In those 

 which arise from one region of the egg, and are predestined not 

 to form the germ cells, the chromosomes break up at the anaphase 

 ot mitosis and leave some unessential pieces of heterochromatin 

 (lacking centromeres) on the equator (Fig. 48). 



hi all these instances the cytoplasm of particular cells is telling the 

 chromosomes what to do. But paradoxically enough, it can only 

 be passing on what it has had from the nucleus. For in polymitotic 

 maize, a recessive gene compels all the pollen grain nuclei produced 

 by a plant homozygous for it to divide again as soon as they are 

 formed, and to go on dividing until the whole nuclear organization 

 is destroyed. In this case it is clear that the gene has acted on the 

 cytoplasm (already in the mother plant, for the effect is not seen in 

 the individual pollen grains of the heterozygote) and the cytoplasm 

 has acted in turn on the nucleus, a different nucleus. 



Finally, it should not be lost sight of that the cytoplasm is in 

 immediate control of all the everyday details of mitosis. Each 

 centromere and nucleolar organizer is a gene, simple or compound, 

 and when all ot them keep time in nuclear cycle they are illustrating 

 the unity of reaction of any species of gene to the changes in the 

 common substrate, the cytoplasm. Other genes whose activities are 

 less minutely observable evidently behave in the same way to give 

 the disciplined uniformity of nuclear propagation from which the 

 authority of nuclear action derives. 



Co-operation and Competition 



The interaction of the cytoplasm and the genes in the nucleus, 

 each modifying the behaviour and activities of the other, is now 

 becoming clear. It expresses itself also in the relations of cells of 

 different genetic character. When, as Barber found in Uvularia, two 

 pollen grains are formed at meiosis, one having extra chromosomes, 

 and the other lacking these same chromosomes, the first survives 

 poorly and the second survives not at all. If, however, the cell-wall 

 that should have separated them is inhibited by a heat-shock, so that 

 the two complementary nuclei are lying in tlie same C)l:oplasm, 



198 



