DOUBLE MONSTERS IN FISHES 71 



Farther than a realization that twins might be due 

 to environmental changes, Gemmill did not go. His 

 attitude is essentially that of a pure morphologist of the 

 old school and not that of an experimenter. 



Stockard, however, taking his cue from my own 

 theory of the cause of twinning in the armadillo, under- 

 took with some degree of success to produce fish twins 

 by lowering the rate of development of the early embryo. 

 He found that by putting recently fertilized eggs of 

 Fundulus into a refrigerator for fairly long periods 

 and then bringing them back to normal temperature, 

 he obtained a few double monsters. Since twins in 

 Fundulus are extremely rare there can be no doubt but 

 that twinning was induced by the cold. Stockard got 

 similar results in a few instances with lack of oxygen. 

 The common factor was, as he calls it, ^'arrested develop- 

 ment." He feels that there is a very intimate relation 

 between twinning and the process of gastrulation, but 

 is rather vague about what this relation is. In one place 

 he says: ''Either stopping development or greatly 

 reducing its rate during cleavage or before the germ 

 ring has formed, that is, at periods preceding gastrulation, 

 frequently serves to cause doubleness in the subsequent 

 embryo formation." In another place he writes: "The 

 origin of two embryonic axes or growing-points on the 

 germ ring of the fish probably results from a rather mild 

 or slight reduction in the normal developmental rate at 

 the time of gastrulation or embryonic shield formation." 

 We are thus left in doubt as to the critical period for 

 twinning. Stockard seems to waver between two 

 positions and it is uncertain whether he considers the 

 critical period before or during gastrulation. He has. 



