FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



331 



development caused by lack of oxygen within the cocoon, which is 

 occasioned by the high mortality of the eggs and consequent 

 foulness. 



A formal explanation of twinning and the replacement of a single 

 axis of polarity by two axes, more or less independent, is to be 

 found in the principle of axial gradients. The maximal suscep- 

 tibility of the apical point of the gradient, when acted on by 



Fig. 153 

 Partial twinning in trout brought about by reduced oxygen supply during pre- 

 gastrulation stages. Left, unequal components, anterior duplication. Centre, 

 anterior duplication, unequal components: component on the left has a very 

 small head and is cyclopean. Right, a subnormal individual, with only one eye, 

 no mouth, gills or tail fin, and much reduced trunk, is attached to the surface 

 of the yolk-sac opposite to the larger normal individual. (Redrawn after Stockard, 

 Amer.Journ. Anat. xxviii, 1921.) 



depressant agencies, brings about the depression of its level of 

 activity below that of the immediately neighbouring regions. These, 

 in all cases where the original embryonic area is a flat plate or 

 blastoderm, as in fish, birds or mammals, will be symmetrically 

 situated right and left of the original apical point. 



Interesting confirmation of the truth of this interpretation is pro- 

 vided by the cases of twinning presented by the Oligochaetes. As 

 mentioned above, these worms are characterised by the presence of 



