38 EARLY AMPHIBIAN DEVELOPMENT 



bilateral symmetry. Furthermore, the plane of symmetry bears no 

 relation to the point of pricking. ^ It is necessary, therefore, to 

 assume that even in the unfertilised egg all the meridians are not 

 perfectly equivalent, and that one of them has some slight differ- 

 ential in respect of the others. This meridional differential, how- 

 ever, must also be supposed to be due to some unequal incidence 

 of external factors operating in the ovary. However this may be, 

 the egg must acquire and possess some feeble determination of a 

 plane of bilateral symmetry which becomes realised in the absence 



I 

 i 



Fig. II 



Cortical localisation of dorsal lip region in frog's egg shown by forced rotation 

 of the egg. Thick line, original plane of symmetry ; chain line, new plane of sym- 

 metry, passing through centre of grey crescent region (stippled) and mass of yolk 

 which has streamed down to lower pole by gravity. (From Weigmann, Zeitschr. 

 f. Wiss. ZooL cxxix, 1927.) 



of any more powerful stimulus, as in the case of artificial partheno- 

 genesis, but which may be overridden by such stimuli as the point 

 of entry of the sperm, or the direction of incident light, ^ or the 

 direction in which the yolk streams down when the egg has been 

 forcibly inverted. In the latter case, the plane of symmetry is 

 determined in such a way as to include the centre of the original 

 grey crescent and the centre of the mass of yolk which has 

 streamed down under the effect of gravity : the dorsal lip of the 

 blastopore therefore arises in the normal position, but the lateral 

 lips form a crescent the concave side of which is always turned 

 towards the mass of yolk, wherever it may be (fig. 11).^ 



^ Bataillon, 1910; Brachet, 1911. - Jenkinson, 1909 a. 



^ Weigmann, 1927. 



