186 CHARLES R. STOCKARD 



The relative conditions of the individual components in the 

 double trout are of considerable importance and will be discussed 

 in connection with their several particular bearings in the pages 

 beyond. 



c. An explanation of the frequent occurrence of twins and double 



chick embryos 



It is extremely rare among birds for a double-headed or other- 

 wise double individual to hatch from the egg ; a few such irregular 

 cases have been recorded. I have never, however, found record 

 of complete twins hatching from the hen's egg. On the other 

 hand, when the earlier stages of chick development are studied in 

 the laboratory, one rarely fails, even in a limited experience, to 

 meet with double and twin embryos. The prevalence of these 

 early specimens has long furnished material for studies on twin- 

 ning in the chick. Among many such investigations are those 

 of Gerlach ('82), Burckhardt, Dareste, Klaussner, Erich Hoff- 

 man, Mitrophanow, five somewhat more recent studies by Kaest- 

 ner ('98, '99, '01, '02, and '07), and most recently the description 

 of several double chick embryos by Tannreuther ('19). 



The almost abundant occurrence of double specimens among 

 the limited numbers of eggs developed in the laboratoiy and the 

 well-known high mortality among incubating eggs of the poultry 

 farm, makes it highly probable that double and deformed em- 

 bryos are not uncommon under natural conditions, but that they 

 usually die during the early days of development. 



From a survey of the literature on double monsters in the 

 various vertebrate classes, it would be impossible to form any- 

 thing like a correct estimate of the comparative frequency of such 

 individuals in these several groups. It would be simply specula- 

 tion to claim that doubleness was more frequent among the em- 

 biyos of birds than among those of mammals. Yet the double 

 condition in birds is just here of particular interest as probably 

 being due to a somewhat definite and uniform cause arising out 

 of their peculiar mode of development. The double bird em- 

 bryos are very probably the result of a rather easily followed 

 natural experiment. 



