What Is Development? 3 



Developmental phenomena are not restricted to higher animals. 

 Even the single cell goes through a corresponding progression of changes. 

 Its life cycle begins when it arises from the division of its parent into two 

 daughters and ends when it in turn divides in two. Between times, it 

 grows in size and protoplasmic content, synthesizes cellular constituents 

 in an orderly and well-regulated manner, and can, indeed, develop new 

 organelles (a term used to designate any of the parts of a cell), and 

 change its form and functioning drastically. Many higher protozoa and 

 algae display complicated changes in this respect, and a few of these 

 cycles will be described in succeeding sections. 



Populations of cells also show developmental phenomena. For ex- 

 ample, a culture of microorganisms has a hfe cycle of its own, as con- 

 trasted with those of the individual cells of which it is composed. As will 

 be described in detail later, when we add an inoculum of bacteria to 

 nutrient broth, the culture experiences an orderly succession of well- 

 defined growth stages, reaches a stationary state and ultimately a period 

 of decline during which the cells degenerate and die. Thus the bacteria 

 en masse act in much the same fashion as the cells of a higher organism 

 except that they are loosely distributed and largely independent, whereas 

 the cells of an animal are dependent upon each other and are compressed 

 into a compact organized structure. In fact, some sophisticated micro- 

 organisms exist for part of their culture cycle as independent cells, and 

 during the remainder of it actually do come together and construct organ- 

 ized multicellular structures. A later chapter will describe these fascinat- 

 ing organisms. 



A little higher on the scale, we come to the primitive animals, which 

 display, on a limited level, the complicated series of developmental events 

 that the higher animals show. One of the most interesting primitive groups 

 in this respect, the coelenterates, will be discussed in detail. 



The Central Problems of Developmental Biology 



As we consider the life cycles mentioned above, three central prob- 

 lems of development become apparent and will be taken up here. 



GROWTH 



The egg starts as a single microscopic cell with one nucleus and by 

 growth and replication produces an adult containing millions of cells. 

 Needless to say, this represents a many-millionfold increase in proto- 

 plasmic mass. A number of questions come to mind when we think about 

 this phenomenon. How is growth initiated? What raw materials are re- 

 quired for the synthesis of protoplasm? What specific chemical reactions 



