76 THE PHYSIOLOGY OF TWINNING 



Dareste, however, made a distinct advance in that 

 he adopted for study the bird egg where, if anywhere, 

 he should have been able to see double monsters in the 

 making. He found a number of cases of two or more 

 blastoderms on a single egg; not only that, but two em- 

 bryos upon a single blastoderm. He noticed that these 

 paired embryos on one blastoderm often lie symmetrically 

 with reference to each other and this to him seems to 

 make the idea of double monsters as fusion products 

 entirely reasonable. On these grounds he adopted the 

 theory that double monsters are the product of the 

 fusion of two embryonic axes that have arisen on 

 a single blastoderm. He favors the theory of the 

 Saint-Hilaires that like part tends to fuse with like, 

 though there are absolutely no grounds for such an 

 assumption. 



As to the causes of twinning Dareste has no theory. 

 One would think that a man so imbued with the idea 

 that monstrosities are all due to developmental disturb- 

 ances might readily have concluded that double monsters 

 were so produced, but he did not seem to take this step. 

 On the contrary, he specifically states that twins and 

 double monstrosities are predetermined before laying 

 through some disturbance of the process of fertilization. 

 For him double monsters are not in the same category 

 with the numerous types of single monstrosity which he 

 describes so much in detail. 



MODES OF TWINNING IN BIRDS 



Since the appearance of Dareste's treatise a large 

 literature concerning double monsters in birds has grown 

 up, and many excellent figures of double monsters of 



