36 EMBRYOLOGY 



This gives a picture of a very labile type of system in which the original 

 organization of the sea urchin egg can be completely changed by the proper 

 chemical treatment. The eggs of other species of invertebrates and of 

 vertebrates exhibit a similar type of organization. 



Twinning and duplication of structures 



Because of this very labile or flexible type of determination in the early 

 egg and because the egg is susceptible to these chemical changes in the 

 environment, we find a number of duplications, or doublings, in embryonic 

 development. And the explanation must lie in the fact that there is a labile 

 organization which is affected by some chemical treatment. Identical twins 

 arise by the splitting of a single egg. It is easy to see how this is possible, 

 since experimentally when an egg is cut into halves two identical embryos 

 are obtained. A less extensive duplication results in Siamese twins in 

 vertebrates, in which the two embryos remain fused at some point. Still less 

 extensive duplication may result simply in two heads joined to a single 

 trunk, or two trunks with a single head. Indeed the twinning process may 

 involve duplication of only a single organ. Animals sometimes develop with 

 double limbs, or even with triple limbs. Finally, parts of a structure like a 

 finger may split during development to give two or more fingers. 



Although duplications are very interesting, they are certainly not desir- 

 able from the point of view of the organism possessing them. Therefore 

 studies on the factors bringing about duplication have been numerous and 

 have led to a theoretical interpretation of this process. 



At the onset it may be stated that in the embryos of some animals doubling 



Fig. 16. A double-headed fish embryo 

 which developed in a normal environment. 

 It possesses four complete eyes, two brains, 

 and two mouths and has a common stomach 

 and tail. Such monsters do not usually sur- 

 vive as adults, although occasionally one is 

 found in nature. 



