2 What Is Developmenf? 



have previously been grown in the presence of glucose as an energy 

 source, to a new sugar, maltose, the cells, in order to utilize it, must first 

 synthesize an entirely new enzyme that splits the maltose into usable 

 fragments. They can do this in a relatively short time. If the maltose 

 were removed and replaced by glucose, the new enzyme would disappear 

 from the cells and no record would be left of this interlude of physio- 

 logical change. 



All such activities have common properties. They do not occur in 

 any regular rhythm but merely represent sporadic adjustments to specific 

 environmental stimuli. They are usually reversible; that is, the progression 

 of changes leaves the organism neither vastly different from what it was 

 before nor unable to return to its former state. 



2. Long-term genetic and evolutionary changes. Alterations in the 

 genetic apparatus of an organism are called mutations and are inherited 

 by the offspring. A single mutation may be minute in itself, perhaps lead- 

 ing only to the loss or gain of the capacity to synthesize a single enzyme, 

 and its effect on the form or functioning of the organism may be corre- 

 spondingly slight. If by virtue of the change, the mutant is more fitted 

 to survive than its unaltered relatives (if it can breed faster, for example), 

 it will flourish and the following generations will probably include a 

 greater proportion of that mutant type. When successive mutations stand 

 the test of natural selection, they can, over many generations, produce a 

 variety of organism greatly different from its distant forebears. In the 

 aggregate, these progressions contribute to the evolution of species. 



Note how different these evolutionary changes are from the short- 

 term changes mentioned before. First of all, a much longer time is in- 

 volved, for the appearance of a new variety of organism in large numbers 

 may require many generations of growth and reproduction. Second, the 

 biologist who studies these phenomena must examine populations, not 

 individual organisms, in order to follow the spread of genetic changes. 



3. Developmental events. Developmental events are seen most dra- 

 matically in the growth patterns of higher animals. The fertilized egg 

 undergoes a very orderly series of changes to become an embryo. The 

 embryo develops further into the young animal, which in turn matures 

 into an adult. The adult reaches its peak and then experiences a series of 

 degenerative changes that ultimately lead to death. At maturity the ani- 

 mal releases eggs or sperm which originate yet another cycle of develop- 

 ment. 



The tempo of these phenomena is neither fast enough to be physio- 

 logical nor slow enough to be evolutionary. Moreover, they are progres- 

 sive; that is, they occur in a regular sequence with little variation, each 

 leaving the organism different from its former state and unable to return 

 to it. They begin before birth and end shortly after death, and in toto 

 they represent the life cycle of the organism. 



