68 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



side. Further, in sub-lethal concentrations, the dorsal regions are 

 the most inhibited in their differentiation.^ 



This last method allows us to make a further statement, namely 

 that the dorso-ventral activity-gradient becomes progressively more 

 intense (steeper) between fertilisation and gastrulation. In just- 

 fertilised eggs, disintegration in lethal concentrations begins at the 

 animal pole and then spreads along the dorsal side : in some cases 

 a second centre of disintegration appears in the region of the grey 



Fig. 28 



Differential susceptibility in a frog's egg exposed to KCN from the 2-cell 

 stage: disintegrated cells are shown light. The animal pole area (central) has 

 disintegrated ; also an area of cells, near the equator on one side, in the future 

 organiser region. (After Bellamy, Biol. Bull, xxxvii, 1919, modified.) 



crescent before the primary disintegration has spread to this area. 

 During cleavage, the susceptibility of the dorsal lip region in- 

 creases, until in late blastulae this region begins to disintegrate 

 before or at the same time as the apical pole. In gastrula stages, the 

 dorsal lip region is always the first to disintegrate.^ It is probable 

 that this process is correlated with the acquisition of organiser 

 properties by the dorsal (grey crescent or dorsal lip) region (fig. 28). 



The method by which bilateral symmetry is determined in the 

 egg of Echinoderms is still problematical- ; but the localisation of the 

 plane can be revealed by susceptibility experiments at a stage before 

 any bilateral symmetry is visible in the embryo.^ 



Further, a labile determination of bilateral symmetry has been 



^ Bellamy, 1919; Bellamy and Child, 1924. 



2 Horstadius, 1928. ^ Child, 1916A. 



