THE POLARITY OF THE EGG OF URECHIS CAUPO

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Abstract
The author in 1931 reported that the indentation which normally occurs in the unfertilized egg of U. caupo marks the animal pole of the egg. After insemination, the indentation disappears and the polar bodies form at the same point. C. V. Taylor, 1931 (See B. A. 6: entry 17471) using the same species, found no mark of polarity previous to formation of the polar bodies, i.e., no relation between the indentation and the point of extrusion of the polar bodies. In the present paper the author shows that in some eggs the first indentation disappears after insemination and a second may form which does not coincide fully with the first; but the point of extrusion of the polar bodies almost invariably coincides with the second indentation. A certain % of the eggs have 2 indentations and the point of extrusion of the polar bodies usually coincides with one of these. In eggs having only 1 indentation which disappears entirely and does not reappear, the indentation marks the pole of the egg; 81% of such eggs showed coincidence between the site of indentation and the point of extrusion of the polar bodies; 11% were 10[degree]-60[degree] off; and 8% were 60[degree]-180[degree] off. The discrepancy reported by Taylor is accounted for by eggs which had 2 indentations, one of which had been overlooked. It is concluded that the polarity of the egg is really determined before fertilization as in most eggs.

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