8o 



THE PHYSIOLOGY OF TWINNING 



that the two inwardly growing embryos have met each 

 other head-on instead of growing past each other as in 

 Kaestner's case. On this account there is a considerable 

 amount of crumpling up of the brains and other anterior 

 parts. The heart of one has been crowded off to the 

 left and that of the other, to the right. Such a duplicity 

 as this cannot have arisen as a fission product of a single 



Fig. 37. — The result of a head-on collision between chick twins 

 that have resulted from double, symmetrical gastrulation on opposite 

 sides of the blastoderm. The results of the collision are seen in the 

 crumpled forebrain and displaced hearts. This case is equivalent in 

 mode of origin to the type of Patiria twin shown in Fig. 7. (Original.) 



embryonic axis. Dareste, Kaestner, and Tannreuther 

 have described cases similar to this, but such purely 

 mechanical fusions as this are quite different in charac- 

 ter from most double monsters, and must not be con- 

 sidered as supporting the theory that cosmobia (truly 

 symmetrical double monsters) are the product of the 

 symmetrical fusion of paired embryonic axes. 



A really crucial case has recently come into my hands 

 through^the kindness of Dr. B. H. WiUier. This is a 



