TWINNING AND DUPLICATION OF STRUCTURES 37 



of the head region is not rare even in a normal environment. One of these 

 double monsters is illustrated in Figure 16. It has two complete heads with 

 four eyes and a common trunk and tail. In the eggs of other forms, such as 

 an armadillo, each egg normally and regularly splits in some way to give 

 four complete embryos, which give rise to four normal adults. Thus this 

 reduplication process is a normal one as well as an abnormal one. 



What factors bring about this splitting in eggs? If certain eggs are 

 treated with chemicals which retard development for a time, or if they are 

 simply cooled for a prolonged period and then returned to their normal 

 environment or their normal temperature, as the case may be, a certain per- 

 centage of the eggs may show a doubling of the head or the trunk region. 

 Several other types of treatment, such as radiation with ultraviolet light, 

 produce similar effects. The eggs must be exposed to the external agent in 

 a very early stage of development, during which they still exhibit a labile 

 organization. Thus, it is thought that any agent which drastically slows 

 development or which stops it for a time will produce duplications, but only, 

 of course, if the eggs survive such drastic treatment. And the mortality in 

 these experiments is usually very high. It is evident that, although twinning 

 and duplications may be produced under laboratory conditions by chemical 

 treatments, under natural conditions such as development in the uterus the 

 underlying cause may arise from the genes. Indeed, in humans polydactyly — 

 that is, a duplication of one or more fingers — is inherited as a dominant gene. 



This theory of twinning, which arose from experiments on whole eggs, 

 may be extended to explain the duplication of parts of the egg. We may 

 suppose that the doubling of a limb is caused by some injurious agent acting 

 on the very early stages of limb development, while this development is 

 still in the stage of labile organization. Furthermore, since every organ 

 passes through a phase in development where it shows some labile organ- 

 ization, we can see the possibility that any organ of the body, if exposed to 

 a retarding agent at the correct stage of development, might show duplica- 

 tion. And this appears to be true. 



How do these retarding agents act? In general, any chemical compound 

 does not affect all parts of the egg to the same extent. Some regions are 

 injured more than others. Thus, if one region is injured severely and the 

 regions on both sides are only slightly injured, these two sides could go on 

 to form an embryo. This is a sort of "splitting" induced by chemical treat- 



