HOW THE CYTOPLASM ACTS 



How this works has been shown by La Cour in the differentiation 

 of the bone-marrow of mammals. Here two opposite types of cell- 

 lineage are separated; one, with high nucleic acid content in the 

 cytoplasm, has rapid divisions of its nuclei, a strongly developed 

 spindle and chromosomes heavily charged and strongly spiralized. 

 This type gives rise to the red blood corpuscles. The other, with 

 low nucleic acid content, has slow divisions, a poor open spindle 

 scarcely capable of executing the anaphase movement, and chromo- 

 somes feebly charged and weakly spiralized. This gives rise to the 

 white blood corpuscles in their far smaller numbers. Thus two lines 

 of development are set going in the same ancestral nuclei by two 

 conditions of the cytoplasm, arising no doubt in different positions 

 in the tissue (Fig. 46). 



The differentiation of the embryo-sacs and pollen grains of 

 flowering plants exactly parallels that of the blood precursors. In 

 the pollen of Angiosperms one of the daughter nuclei is pressed by 

 the first mitotic spindle against the wall, while the other is left in 

 the middle of the cell. The peripheral, or generative, nucleus forms a 

 small cell and is rich in nucleic acid: it divides again to give the 

 two condensed sperm nuclei, often before the germination of the 

 grain. The central, or vegetative, one forms a large cell and is poor 

 in nucleic acid : it becomes large and diffuse and loses first its staining 

 power, and then its coherence. It vanishes without further mitosis. 

 (The nucleus of the ripe red blood corpuscle, similarly drained of 

 nucleic acid, similarly dissolves.) This differentiation must depend on 

 the fact that the distribution of materials in the pollen grain before 

 mitosis is not uniform. There is a gradient and the position of the 

 mitotic spindle is adjusted to lie along this gradient. If the axis of the 

 spindle lies crosswise to the normal (following heat shock) cells and 

 nuclei with similar properties are produced: differentiation fails 



(Fig- 47). 



In a species of Sorghum extra heterochromatic chromosomes 

 have been found to cause the vegetative, as well as the generative, 

 nuclei to divide again. Extra nucleic acid, which the additional 

 chromosomes make available in the cytoplasm, stimulates nuclear 

 division in a way which recalls the plan of development of the 

 gametophyte in the lower plants. In extreme cases the nuclear 

 division may give four or five generative nuclei and thereby kill 



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