go THE PHYSIOLOGY OF TWINNING 



the relatively marked prevalence of katadidymi among 

 the birds, for this is the commonest expression of twin- 

 ning in the group, in striking contrast with the situation 

 in the fishes where anadidymi are the almost universal 

 type and katadid3ani extremely rare. 



All of these facts accord with the phenomenon pre- 

 viously referred to: that in the birds developmental 

 interruption at the time of laying occurs only after the 

 process of gastrulation has made considerable progress. 

 We would expect, then, and actually do find that twins 

 from separate blastoderms are very infrequent, that 

 those derived from two gastrulations are next in rarity, 

 that anadidymi come next, and that katadidymi are the 

 most frequent. 



