82 



THE PHYSIOLOGY OF TWINNING 



a normal single condition. We may conclude then 

 that true double monsters {cosmohia) cannot he derived 

 through the coming together at their anterior ends of separate 

 embryonic axes. 



In this category of twins we may include the very 

 rare case of Dareste (Fig. 39) consisting of a set of triplets 



upon a single blasto- 

 derm. The facts 

 that the heads all 

 point inward to- 

 ward the center and 

 that each lies on its 

 left side like a nor- 

 mal single embryo, 

 argues for their deri- 

 vation from three 

 separate and equiv- 

 alent points of 

 gastrulation. No 

 other similar condi- 

 tion has ever been 

 described. 



3. A third type 

 of twin chick em- 

 bryo is that which has obviously arisen through the 

 separation of the right and left primordia of a single 

 embryonic axis. Embryos of this sort are not at all 

 uncommon. One case of Tannreuther's that seems to 

 me to bear this interpretation is that of a three-somite 

 stage in which there are two entirely separate heads 

 and the body, back to the posterior end of what was 

 originally the head process, is symmetrically divided into 



Fig. 39. — A fine example of identical 

 triplet chick embryos derived from triple 

 gastrulation of a single blastoderm. (After 

 Dareste.) 



