22 THE PHYSIOLOGY OF TWINNING 



gastrulae with equivalent and symmetrical archentera. 

 If the fission is somewhat unilateral or asymmetrical, 

 as it often is, twin archentera of different sizes occur and 

 the larger often becomes dominant over the smaller, deter- 

 mines the axis of the embryo, and absorbs the smaller. 



TWINNING A RESULT OF A LOSS OF POLARITY 



Physiologically considered, what happens in aU these 

 cases is this: A lowering of the developmental or meta- 

 bolic rate of the embryo, either before or during cleavage, 

 has to a more or less complete extent obliterated the 

 original polarity, which has been shown to depend on a 

 gradient of oxidative and other activities running from 

 the animal or apical pole to the vegetal or basal pole 

 of the egg or embryo. The rate of metabohsm of the 

 whole is lowered to such an extent that in extreme 

 cases the whole gradient is obliterated, with the result 

 that no point is distinctly apical to any other; so that 

 any point may acquire independence and start to 

 invaginate if stimulated so to do. If the original 

 gradient is only partially lost certain secondary basal 

 regions may become isolated and begin independent 

 invagination. If the original basal region merely under- 

 goes fission we get twin or equivalent archentera. 



I look upon the process of normal gastrulation as a 

 condition much like the formation of a new zooid in a 

 planarian. The ingrowth of the archenteron at the 

 point most distant from the original apical end of the 

 embryo is due to a physiological isolation of a new 

 actively growing region that is highly susceptible to 

 growth disturbances. It is the ingrowth of the archen- 

 teron that establishes the new axis of symmetry and, 



