. The elements of experimental embryology. Embryology, Experimental; Embryology. A PRELIMINARY EXPERIMENTAL ANALYSIS 37 the thread. The result of the experiment can be checked by cutting the egg into sections, for the path of entry of the sperm is indicated by a trail of pigment leading into the interior of the egg, and the grey crescent which indicates the dorsal meridian can also be identified by the retreat of pigment from the surface By this means it can be proved that the grey crescent and therefore the mid- dorsal line is normally opposite or nearly opposite to the point of entry of the
. The elements of experimental embryology. Embryology, Experimental; Embryology. A PRELIMINARY EXPERIMENTAL ANALYSIS 37 the thread. The result of the experiment can be checked by cutting the egg into sections, for the path of entry of the sperm is indicated by a trail of pigment leading into the interior of the egg, and the grey crescent which indicates the dorsal meridian can also be identified by the retreat of pigment from the surface By this means it can be proved that the grey crescent and therefore the mid- dorsal line is normally opposite or nearly opposite to the point of entry of the sperm. If, as sometimes happens, two sperms enter an egg simultaneously, the grey crescent is determined relatively. Fig. lo Diagrammatic equatorial sections through dispermic frogs' eggs, showing that the grey crescent (position of which is indicated by thin outHne) is formed opposite the midpoint between the two points of sperm-entry. The plane of symmetry is indicated by a broken line. (From Herlant, Arch, de Biol, xxvi, 1911, figs, ix, X, p. 250.) to them both, and arises antipodally to the meridian half-way between their two points of entry. ^ The second step in differentia- tion, the acquisition of bilateral symmetry, is therefore also deter- mined mainly in relation to a factor external to the egg (fig. 10). But, as is very often found in the study of development, the main determining factor is not the sole one capable of exerting an effect. This conclusion is necessitated in this case by studying partheno- genetic eggs. Artificial parthenogenesis may be induced in the egg of the frog by pricking it with a needle dipped in blood or lymph. There is then no point of sperm-entry, and yet the eggs develop ^ Roux, 1887; Jenkinson, 1909 a; Herlant, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Huxl
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