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, 

 191 1, 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, 191 1. 



