The cell in development and inheritance . ME 355 piece (p. 212), which is itself genetically related to the centrosome ofthe last cell-generation (p. 170). These facts seem explicable onlyunder the assumption that in these cases the centrosome, or a sub-stance which it carries, gives an active stimulus to the cytoplasmwhich incites the aster-formation about itself, and in the words ofGrifhn disengages the forces at work in mitosis (96, p. 174). Forthese reasons I incline to the view that in the artificial aster-formationdescribed by Morgan ^ the centrosomes there observed should not beregarded
The cell in development and inheritance . ME 355 piece (p. 212), which is itself genetically related to the centrosome ofthe last cell-generation (p. 170). These facts seem explicable onlyunder the assumption that in these cases the centrosome, or a sub-stance which it carries, gives an active stimulus to the cytoplasmwhich incites the aster-formation about itself, and in the words ofGrifhn disengages the forces at work in mitosis (96, p. 174). Forthese reasons I incline to the view that in the artificial aster-formationdescribed by Morgan ^ the centrosomes there observed should not beregarded as the creations of the asters, but rather as local depositsof material which incite the aster-formation around them. That thecentrosomes or astral centres are centres of division (whether activeor passive) is beautifully shown by Boveris interesting observationson partial fertilization referred to at page 194. Again, Boveri has observed that the segmenting ovum of Ascarissometimes contains a supernumerary centrosome that does not enter. Fig. 165.— Eggs oi Ascaris with supernumerary centrosome. [BOVERI.]A. First cleavage-spindle above, isolated centrosome below. B. Result of the ensuing division. into connection with the chromosomes, but lies alone in the cytoplasm(Fig. 165). Such a centrosome forms an independent centre of divi-sion, the cell dividing into three parts, two of which are normalblastomeres, while the third contains only the centrosome and attrac-tion-sphere. The fate of such eggs was not determined, but theyform a complete demonstration that it is in this case the centrosomeand not the nucleus that determines the centres of division in thecell-body. Scarcely less conclusive is the case of dispermic eggs insea-urchins. In such eggs both sperm-nuclei conjugate with the egg-nucleus, and both sperm-centrosomes divide (Fig. 166). Thecleavage-nucleus, therefore, arises by the union of three nuclei andfour centrosomes. Such eggs divide at the first cleavage into four
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
