EMBRYOLOGY 



PRESUMPTIVE EYE 



CORNEA 



LENS 



YOUNG EGG EYE REGION 



NO VISIBLE STRUCTURES ONE WEEK LATER 



Fig. 1. The amphibian egg at the left is in an early stage of development. The 

 many cells of the region labeled presumptive eye" resemble each other so 

 closely that no differences can be detected. These same cells one week later have 

 become rearranged into a well-developed eye. at the right. Some cells have 

 formed the retina of the eve. others the lens, still others the nerve fibers of the 

 eye. What caused cells, originally alike to all appearances, to become so differ- 

 ent in these different structures? 



facts as scarce. as they were during the seventeenth, eighteenth, and nineteenth 

 centuries, it is not surprising that theories of development were rather naive. 



Two theories were formulated during the seventeenth century. One theory 

 contended that development was simply a process of increase in size. That is. 

 the embryo — the young adult, if you wish — was preformed in all details in 

 the egg. This theory of preformation states that the embryo is already present 

 in the single cell, in the egg. All that happens during development is an 

 increase in the size of the embryo and its parts. Thus, there is no need to 

 explain any real development, any change in complexity. Development is 

 purely a growth phenomenon, an increase in size. 



Such a point of view, logically extended, resulted in the encasement 

 theory. This theory extended the preformation theory to previous generations, 

 reasoning that if an egg contained a small young adult. A. somewhere within 

 A there was a smaller egg containing a still smaller young adult for the 

 next generation. The series of young adults within young adults could be 

 extended back in time to an infinite series. Because of the absurd nature of 

 this theory other embryologists began to examine eggs and began to see 

 that there were no young adults in eggs, that even under the most powerful 



