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By Chr. P. Raven and G. Kerkut (Auth.)

ISBN-10: 0080113435

ISBN-13: 9780080113432

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16). Other sub­ stances, such as sodium thiocyanate, have the opposite effect; they reinforce the animal gradient at the expense of the vegetative gradient so that the animal differentiation of the embryo is promoted, and its vegetative differentiation inhibited. If the polarity of the egg is determined by axial gradients, disturbance of these gradient systems may be expected to cause changes in polarity. We have mentioned above (p. 34) that the structure of the egg can be profoundly modified by centrifuging.

These experiments prove that Weismann's theory cannot be maintained. Development is not due to a qualitatively unequal distribution of a spatial system of developmental factors, localised in the nucleus of the fertilised egg. We must conclude that the spatial multiplicity of the later organism is not preformed as such, either in the cytoplasm or in the nucleus of the fertilised egg. Therefore, development appears to involve an increase in extensive multiplicity. CHAPTER IV Polarity and symmetry; The cortical field In the previous chapter we have seen, that, in broad outlines, the newly fertilised egg is to be regarded as a homogeneous system, and that its multiplicity is only intensive, not extensive.

In several species, the egg can be activated only in the presence of these ions in sufficient quantity (Pasteéis, 1938; Moser, 1939). Heilbrunn (1937) holds the view that the egg cortex consists of a protein-calcium compound, which can be broken down by the action of various stimuli. The liberated calcium is then taken up by the internal cyto­ plasm of the egg. This causes an increase in the viscosity of this region, while the cortex simultaneously liquefies. These processes are considered to be the real cause of activation.

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An Outline of Developmental Physiology by Chr. P. Raven and G. Kerkut (Auth.)

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