4 What Is Development? 



lead to the formation of nuclei, enzymes, cell walls, etc.? From where does 

 the organism draw the energy needed to carry out these syntheses? Why 

 does growth stop? 



Another significant attribute of growth is the fact that all the parts 

 of an organism increase in a carefully regulated manner. That is, if a cell 

 doubles its size, the size of the nucleus, cytoplasmic constituents, cell wall, 

 etc., all increase in proportion to one another. When a boy grows into a 

 man, his organs increase harmoniously. Thus the length of his arm bears 

 a fixed relation to his height, and within rather close limits this same rela- 

 tion holds in all other boys. There must, therefore, be regulatory mech- 

 anisms in cells and organisms that govern such relationships. What are 

 these mechanisms? 



CELLULAR DIFFERENTIATION 



The millions of cells descended from the original fertilized egg are 

 not all the same. Some are skin cells, others are nerve cells, still others are 

 muscle cells, etc. They look different and perform decidedly different 

 functions. When we think about the reproduction of, say, a bacterium or 

 a rabbit, we normally expect the descendants of each to look and act very 

 much hke if not the same as the original bacterium or rabbit. Such is not 

 the case, however, for the descendants of developing cells in an embryo. 

 The processes by which these cells become specialized are collectively 

 called cellular differentiation. These processes are still largely unknown 

 and present a major challenge to developmental biologists. 



Another mysterious aspect of cellular differentiation is the way in 

 which the numbers of each cell type are carefully regulated. For example, 

 you will never find one embryo with many brain cells and few liver cells 

 or vice versa. Here again, as in the case of the boy and his arm, the vari- 

 ous tissues and organs of the body bear a relatively fixed relation to one 

 another. The mechanisms that control the proportions of differentiated 

 cells are still to be discovered. 



MORPHOGENESIS 



A multicellular organism is not simply a bag of cells thrown together 

 helter skelter. It is composed of organs and tissues, and, as has been men- 

 tioned, the parts of the animal are arranged in a specific pattern and bear 

 definite relations to one another in terms of size and cellular content. 

 Morphological regulation extends to the individual cell and its organelles. 

 The establishment of this pattern and the processes whereby the adult 

 organism takes its final shape is termed morphogenesis and requires ex- 

 planation in physical and chemical terms. 



