DLVLLUPMLNT AND DIl FERENTI ATION 



How the Cytoplasjn Acts: Gradients 



The genes thus express tliemselves by what they give to the 

 cytoplasm. But they vary in how quickly they give it; and it varies 

 in how soon it finds something to react with. Genes express them- 

 selves consequently at different times in the course of development. 

 Wc next have to see what the cytoplasm gives to the nucleus and 



Fig. 46. — Blood precursor cells from the bone marrow of a man with pernicious 

 anaemia, the normal contrast in nucleic acid charge between reds and whites being 

 exaggerated. Left, over-charged red with thick chromosomes and over-developed 

 and multipolar spindle which will give several non-viable hypo-diploid cells. Right, 

 under-charged white with long thin chromosomes and undcr-dcvclopcd open spindle 

 which will give a single tetraploid cell by failure of anaphase. X 2,500 (after 

 La Cour, 1944). 



how this varies with development. The control of the nucleus by 

 the cytoplasm is most readily seen in the synchronization of mitosis 

 (and meiosis) in groups of cells. Such a synchronization is found in 

 all tissues where cell walls are poorly developed or absent. In the 

 cleaving animal egg, and within follicles of the testis or anther, in 

 the endosperm, in the twin diploidized cells of fungi, and even in 

 the feebly-walled pollen grains in the orchids, all the mitoses move 

 in step. They do so, apparently, because the supplies of materials 

 necessary' for mitosis, especially proteins and nucleic acid precursors, 

 are uniformly diffused in the cytoplasm of the whole group of cells. 



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