TWINNING IN BIRDS 89 



ning, the period of gastrulation, has passed in most of 

 the eggs before they are laid. Only a few eggs that 

 are laid prematurely, before the process of gastrula- 

 tion has been completed, undergo twinning. Patterson 

 (1909) has shown that there is considerable variability 

 in the state of advancement of eggs at the time of 

 laying. Certain hens have a greater tendency than 

 others to deposit eggs before gastrulation is complete. 

 If this trait is hereditary it would not make much prog- 

 ress in the race because the prematurely laid eggs are 

 very likely to develop non-viable double monsters or 

 other abnormalities. By a process of natural selection 

 it has probably resulted that only strains of birds with 

 an inherited tendency to lay the eggs relatively late have 

 survived. 



Although we have no definite data on the artificial 

 production of twins in birds the prevalence of twins 

 reared under artificial conditions implies that the twin- 

 ning process is the result of some interruption of the 

 normal course of embryonic development that results 

 in the partial deaxiation of the blastoderm and thus 

 permits of double gastrulation or else causes the physi- 

 ological isolation of the bilateral primordia of a single 

 embryonic axis. The fact that the vast majority of bird 

 twins are of the conjoined rather than of the separate 

 type leads to the conviction that the former are as a rule 

 the product of a relatively late developmental interrup- 

 tion involving the final steps in the establishment of 

 the embryonic axis. An earlier retardation would be 

 expected to cause the fission of the head process and a 

 later retardation that of the primitive streak or posterior 

 parts. This would seem to be a rational explanation of 



