72 THE PHYSIOLOGY OF TWINNING 



however, rendered a distinct service to our understanding 

 of the causes of twinning by his experimental demonstra- 

 tion that twins may ht produced experimentally by 

 lowering the developmental rate. Just why some eggs 

 twin while others remain normal and still others in the 

 same batch exhibit various types of single deformity 

 we do not at all know. We have known for a long time, 

 however, that eggs are highly variable in their suscepti- 

 bilities to various inhibiting agents. It probably requires 

 just a certain degree of susceptibility in an egg and a 

 certain degree and duration of inhibition to give the 

 particular result we call twinning; and just the right 

 combination of the two variables does not often occur in 

 Fundulus. 



Evidently in trout the frequency of twinning is 

 greater, but even there the percentage of twins is very 

 small, for there is hardly more than one twin in i,ooo 

 eggs under the conditions prevailing in fish hatcheries. 

 Doubtless, however, if Stockard's methods were tried 

 on the trout or the salmon, relatively large numbers of 

 twins would be produced. 



