230 



EMBRYOLOGY 



lining of the uterus. Adding all these cell divisions together, we see that the 

 adult body is producing vast numbers of cells every day. 



Even on a chemical basis the adult organism is a very labile sort of 

 system. If animals are fed an element which is labeled in some way — such as 

 radioactive phosphorus in the form of phosphate — you find that the inorganic 

 phosphate very rapidly becomes incorporated into all the complex organic 

 phosphates in all parts of the animal. There must be an equilibrium between 

 practically all the chemical compounds in the adult, with the result that 

 complex chemical compounds are broken down and resynthesized in a 

 dynamic equilibrium. This concept should not be startling, as it is only an 

 extension of the ordinary equilibrium concept of inorganic chemistry. 



Since the organism must be considered as being in a state of dynamic- 

 equilibrium, it is not surprising to find extensive developmental processes 

 occurring in the adult organism. Some animals in the adult stage still retain 

 much of the labile organization of the early embryo, and these animals are 

 able to form two adults from a single adult. If a planarian worm is cut into 

 two parts, each part will develop into a whole planarian, just as an egg is 

 able to form two complete embryos. Other animals are able to re-form a part 

 which is removed, just as the embryo is able to replace parts which have 

 been cut away from it. 



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Fig. 155. Regeneration in a 

 planarian worm. Left: The head and 

 tail are removed and the body cut 

 into five pieces of equal length. 

 Center: Each piece begins to re- 

 generate and forms a small but com- 

 plete worm. Each worm grows to 

 adult size. Note that each of the 

 five pieces retains the same polarity 

 it had in the whole worm. Why is 

 it that at any level an anterior cut 

 surface A forms a head, whereas 

 the posterior cut surface P develops 

 into a tail ? 



