580 



CONTINUITY OF LIFE 



repeated, and this can be continued indef- 

 initely, apparently forever, without change 

 in the cells. Therefore, cells in isolated tis- 

 sues grow like individual protozoan cells. 

 But how do these cells grow when they are 

 a part of the intact embryo? Do they follow 

 these same phases as we have seen in Pro- 

 tozoa and tissues? 



Just as we note limiting factors (nutri- 

 ents and accumulation of wastes) in the 

 studies of Protozoa, we also note limiting 

 factors in the growth of the whole embryo. 

 We know that animals grow to a certain 

 size and stop; they do not grow forever. 

 Something about the community of cells 

 limits the number of divisions that occur. 

 The individual organs also fail to grow 

 after a certain number of cell divisions or 

 after a certain time has elapsed. For ex- 

 ample, if bits of tissue are removed from 

 embryos at various ages it will be found 

 that those from older embryos will not grow 

 as well as tissues from younger ones; in fact, 

 some, like nferve tissue, will not grow at all 

 when removed from adult animals. Others 

 like epithelium retain the capacity for 

 growth throughout the life of the organism. 

 Still others, like cancer tissue, grow readily 

 no matter what their source or age. From 

 these experiments we can conclude that 

 cells, when confined to an organism, are 

 subject to factors that limit growth, and 

 these factors may be the same as the ones 

 that limit growth in a culture medium. 

 What all of these limiting factors are is a 

 fertile field for research. 



In the growth of an embryo, the sizes of 

 the various organs increase at varying rates. 

 The human heart, for example, approxi- 

 mates the growth of the body as a whole, 

 whereas the brain grows tremendously dur- 

 ing the first five years of life and thereafter 

 grows very little. Because all of the organs 

 grow at different rates the general over-all 

 proportions of the human body gradually 

 change. The change is radical during the 

 gestation period (Fig. 23-17) but even after 

 that it progresses slowly throughout life 

 (Fig. 2.3-18). 



In review, we have seen that the complex 

 adult is a result of a long series of changes 

 in which each change precipitates further 

 change, and in which each change occurs as 

 a result of a previous one. The egg is stimu- 

 lated to divide by the impetus given it from 

 the sperm. Subsequent cleavages are ac- 

 companied by the formation of an organizer 

 that stimulates the formation of the embry- 

 onic axis which in turn orients the future 

 embryo. Other stimuli arise, each producing 

 one organ system after another, until the 

 embryo comes into being as an integrated 

 whole. Through growth, intimately tied up 

 with development, the embryo finally be- 

 comes an adult organism. Future studies 

 in embryology must be centered around the 

 nature of the stimuli which bring about 

 these changes. With an answer to this prob- 

 lem will come a method of control of de- 

 velopment. 



