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

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A f t e r Herbst. cleavage and development of the embryo are oriented with regard to the original polarity, and are independent of the direction in which the centrifugal force has been operating. Evidently, the factors governing polarity have not been shifted by the centrifuging; they must be inherent in a component of the egg that is not moved by the centrifugal force. Presumably this is the more solid outer layer of the egg, the cortex. Polarity, then, depends on factors localised in the egg cortex; the axial gradients arise only as a result of these factors.

Now it is the task of science always to look for the simplest possible explanation, and to accept this as long as it has not been disproved. Therefore, we must reject Driesch's hypothesis so long as no more compelling arguments can be advanced in favour of it, and we must accept the conclusion that, on the whole, the egg cytoplasm is a homo­ geneous system with intensive multiplicity only. In the Introduction it was pointed out that the various animal groups by no means behave identically in their development.

After Bierens de H a a n . done the same experiment in newts. After removal of the envelopes, newt eggs at the two-cell stage often assume a dumb-bell shape. When one was placed crosswise on top of the other, two of these dumb-bells often fused into one single germ. Each pair of opposite quadrants of such an embryo was derived from one of the original eggs. Part of the products of these fusions proved to be able to develop into single, more or less har­ moniously built embryos. This was possible even when the fused eggs belonged to different species of newts so that the resulting embryo was a "chimera", consisting of quadrants which alternately belonged to either of the two species (Plate II).

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