592 ON INCREASE IN COMPLEXITY [pt. hi 



a critical concentration susceptibility varies directly as the physio- 

 logical activity, while below this concentration the reverse of this 

 relation is seen, in that regions of higher activity recover and adjust 

 themselves to the reagent more successfully than regions of lower 

 activity. "In applying the susceptibility method to embryonic de- 

 velopment, lethal concentrations may be used but not allowed to 

 act long enough to produce death in the embryo, and in such cases 

 they will, according to Child's interpretation, inhibit regions of 

 higher activity to a more marked degree than regions of lower 

 activity ; while on the other hand, in very low concentrations of the 

 reagent such as to permit acclimatisation and recovery, the region of 

 higher activity will be inhibited, according to Child's interpretation, 

 less than regions of lower activity." A simple instance of the operation 

 of Child's conceptions is the case of the eggs of some polychaete 

 worms, Chaetopterus and Nereis, which, when placed unfertilised in 

 lethal solutions of cyanide, exhibit a progressive dissolution from the 

 anterior end. As development proceeds, the region of maximum 

 susceptibility shifts round to the posterior region (where growth is 

 most active) so that, when the larva is ready to metamorphose, 

 the posterior region is the region which succumbs most readily to 

 lethal and recovers most readily from sub-lethal concentrations of 

 cyanide. 



Child's work on the eggs of the starfish, Asterias forbesii, brought 

 out the fact that the susceptibility gradients in the unfertilised egg 

 were connected in some way or other with the mode of attachment 

 of the individual egg to the parent body in the ovary, obviously a 

 very important point for the question of the origin of polarity and 

 axial symmetry in the developing embryo. Wilson & Matthews 

 showed that the region where the nucleus lay nearest the surface 

 became in the starfish's tgg the apical or animal pole, and Child 

 found that it was from that point on the surface of the egg that dis- 

 integration began. From the behaviour of nuclei which had been 

 extruded during cytolysis in cyanide. Child concluded that the 

 nuclear susceptibility gradient ran in the same direction as the 

 cytoplasmic susceptibility gradient. The direction of the axis, he 

 concluded, was determined by the eccentricity of the nucleus. Child 

 found that during the earlier cleavage stages the general gradient 

 was obscured to a great extent by individual gradients in the blasto- 

 meres and other incidental factors, and that "in the early spherical 



