124 THE PHYSIOLOGY OF TWINNING 



operation of the other. The second t3rpe of double 

 monster was due to the mechanical head-on collision of 

 two embryonic axes as in Figure 37. So far as I am 

 aware there are no cases of human double monstrosity 

 that are to be explained as due to crude mechanical 

 fusions of the latter sort; hence it would appear that 

 all cosmobia are to be explained as products of fission or 

 physiological isolation of the bilateral primordia. Janus 

 monsters, with their two faces and with brains partially 

 double and partially fused, but with bodies more com- 

 pletely double (see The Biology of Twins ^ Fig. 2,6), are to 

 be viewed as instances of the fission of the head process 

 and of the primitive streak in which the latter was more 

 complete than the former. The cyclopian monster shown 

 in a of the same figure is probably another illustration of 

 the independence of the two twinning regions. The bodies 

 are completely isolated except for external fusions, while 

 the head is not double at all, but just the opposite in 

 that even normally paired structures such as the eyes 

 are single. The same factor that produced cyclopia in 

 the head region has evidently produced twinning in the 

 secondary growing region, the primitive streak. This is 

 rather a striking confirmation of the theory that twinning 

 and single monstrosities are due to the same cause- 

 retarded development. A very large percentage of 

 human double monsters are classed as anadidymi, in 

 which the anterior parts are more double than the 

 posterior parts. This was even more strikingly the case 

 for fish double monsters. The anadidymi represent the 

 standard type of vertebrate double monstrosity and it 

 is toward their explanation that most of the theories of 

 the past havejbeen directed. 



