78 



THE PHYSIOLOGY OF TWINNING 



case of triplets, two embryos derived from one blasto- 

 derm and one from the other. The twins, seen in the 



lower part of the figure 

 are believed to have 

 been derived from one 

 blastoderm as the result 

 of double gastrulation. 

 Though these two em- 

 bryos are in all proba- 

 bility the result of two 

 secondary areas of gas- 

 trulation and are there- 

 fore bilateral equiva- 

 lents, one is considerably 

 more advanced than the 

 other. Such examples 

 of unilateral asymme- 

 try are no more difficult 

 of explanation than are 

 cases in which one side 



Fig. 35. — An early stage of twinning 

 in the chick, evidently the product of 

 an unequal fission of an early blasto- 

 derm. (After Kaestner.) 



of a single embryo develops more rapidly than the other. 

 A clearer case of twin chicks derived from two separate 

 blastoderms is one presented by Kaestner (Fig. 35). 

 This case consists of a pair of twins in the primitive 

 streak stage. Here we have a good example of a blasto- 

 derm which has undergone bilateral fission so as to form 

 two rather unequal blastoderms. The fact that the 

 two embryos, though of very unequal size, bear a 

 mirror-image relation to each other, is significant. 



2. Separate twins arising from two points of gastrula- 

 tion on a single blastoderm are usually symmetrically 

 placed so that the anterior ends of the axes tend to 



