128 



THE PHYSIOLOGY OF TWINNING 



separate placenta. This is in contrast with the fact 

 that some human twins, especially double monsters, 

 have a common amnion; and one-egg twins always have 

 a common discoid placenta. 



On the basis of his studies of this embryo, however, 

 Streeter proposes a theory of human twinning which is 



Fig. 49. — Schematic drawing, showing Streeter's idea of the forma- 

 tion of a human one-egg twin. The stages are drawn to the same scale 

 of enlargement so that they may be directly compared. A. Stage corre- 

 sponding to the Miller specimen, showing a hypothetical twin budding off 

 from the primary embryonic node. B. Stage corresponding to the Bryce- 

 Teacher specimen. C. The Mateer specimen. The relatively small size 

 of the twin in this specimen, and the detachment of the yolk sac from the 

 amniotic vesicle are indications of arrest in development. (From Streeter.) 



