38 



EMBRYOLOGY 



SEVERE INJURY 



B \LESS INJURY 



Fig. 17. A theory of duplication in eggs. Left: An injurious agent produces 

 a severe injury at C (shaded area) but only slight injury at A and at B. Right : 

 When the injurious agent is removed, A and B recover faster than C and the 

 development of a two-headed embryo results. 



merit. In Figure 17 region C is assumed to be injured to the greatest extent. 

 Regions A and B, on the other hand, on both sides of this region, are injured 

 only slightly. Now if this egg is returned to its normal environment, A and 

 B, being only slightly injured, will recover and develop, and each may form 

 a complete embryo — or, if we are speaking of an individual organ, each 

 may form a complete organ. 



Thus, throughout development the determination of structures, like the 

 determination within the egg, must be a labile one. That is to say, when the 

 limb arises in the embryo, it must be labile enough so that it can split into 

 two limbs. And so the twinning effect becomes a rather general phenomenon 

 in development, beginning with the egg and going on all the way through 

 to development of the specific structures. 



The phenomena of twinning and of duplication of structures lead to 

 inferences regarding those forms which are not susceptible to experimental 

 treatment and for which observation is the only means of study. For example, 

 it is not possible to experiment with the human egg and embryo. However, 

 since identical and Siamese twins occur naturally in humans, we infer that the 

 organization of the human egg is similar to that of the sea urchin. 



