CHAPTER VI 



TWINNING IN BIRDS 



There is a voluminous literature on twinning (dupli- 

 city) in birds. The egg of the domestic fowl has for 

 a long time been a classic object for the study of verte- 

 brate embryology and many thousands of embryos have 

 been incubated and examined every year in all parts of 

 the civilized world. Doubtless every embryologist who 

 has handled any considerable number of chick embryos 

 has encountered one or more cases of duplicity. As a 

 result of this, numerous papers have been pubHshed 

 about double monsters in the chick, and a great many 

 examples of various kinds of duplicity have been figured. 



THE WORK OF DARESTE 



The classic treatise on double monsters in birds is 

 Dareste's Production Artificielle des Monstruosites (189 1). 

 This volume marks a new era in the scientific study of 

 teratology, for the method is thoroughly experimental. 

 By manipulating the temperature, humidity, and oxygen 

 conditions at various stages of embryonic development 

 he produced all of the regulation types of single monstros- 

 ity and developed a theory of teratology which has been 

 adopted and extended within recent years by Stockard. 

 He believed that the great majority of monstrosities are 

 developmental arrests in the sense that certain organs at 

 critical periods in their development are permanently 

 halted and fail to reach the normal definitive condition. 

 Each organ or system has some especially susceptible 



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