64 THE PHYSIOLOGY OF TWINNING 



of the starfish again to make this clear. Under normal 

 conditions the point of gastrulation is exactly at the 

 vegetal pole or the posterior end of the primary axis and 

 this point is fixed, in all probability, before cleavage begins. 



c) If one wishes to stretch the concept of budding 

 so as to make it include the process of gastrulation it 

 might be legitimate to refer to additional archentera or 

 points of gastrulation as accessory buds. Personally, 

 however, I see no reason for viewing as a process of 

 budding the ingrowth of the archenteron. One might 

 equally readily consider any other point of rapid growth 

 as a bud. If we are unwilling to accept the term budding 

 for the normal process of gastrulation, it seems hardly 

 feasible to use this term for additional centers of gastru- 

 lation. 



d) The question now arises as to whether all fish 

 twins, as Gemmill and Stockard believe, result from 

 separate points of gastrulation (embryonic axes). It 

 seems to me highly probable that many, possibly all, 

 twins which are separate at the two extremities and 

 united only in the middle region by external connections, 

 do actually arise from the physiological isolation of 

 separate embryonic shields. If the original polarity of 

 the blastoderm be largely obliterated two secondary 

 growing-points of equivalent value, or possibly of slightly 

 different values, may arise, neither one of which is domi- 

 nant over the other. From such twin axes will develop 

 the types of twin fishes which we have previously classed 

 as separate twins. If, however, the original polarity 

 is only slightly weakened so that a secondary area of 

 gastrulation arises at the opposite side of the blastoderm, 

 such an area is likely to form a small embryonic shield 



