formed in the embryo, and that they influence the growth and differentia- 

 tion of tissues and organs. At later stages, as we all know, further develop- 

 ment is influenced by exercise, food, sleep, disease, and other factors. But 

 in the early stages, when definite organs are already recognizable, some of 

 these, by producing hormones, influence further development. 



In Brief 



There has never been a clear demonstration of "spontaneous" genera- 

 tion; all plant and animal individuals are assumed to have originated from 

 previous life. 



Every organism starts life as a single cell. 



The single cell from which the complex individual develops has in it 

 all the potentialities of the individual, but probably has not structures cor- 

 responding to all parts of the adult. 



In each species the development proceeds through fairly consistent stages, 

 which are sometimes very distinct. 



Groups of species are remarkably similar in the early stages of develop- 

 ment, although quite distinct later. 



In some respects each individual recapitulates, in his own development, 

 the history of the race. 



The rate of growth and the longevity of a cell vary with the specific 

 nature of the protoplasm of which it is composed. 



The parts of a developing embryo influence one another, probably 

 through the formation of specific chemical substances. 



EXPLORATIONS AND PROJECTS 



1 To find out how fast plants grow, and what parts grow most rapidly, mark 

 growing plants at equal intervals and watch for alterations of levels. Plant seeds 

 of sunflower, beans, corn or tomatoes and grow to maturity. Place India-ink 

 marks on stems from time to time as stems elongate. When seedlings are only 

 two or three inches tall, make marks i inch apart; later use 1-inch intervals. Sum- 

 marize results to answer the questions raised. Record growth-differences as indi- 

 cated by changes in the relative positions of ink spots. Measure the height of the 

 plant periodically and plot its growth. 



2 To investigate the development of the chick embryo, incubate fertile eggs, 

 and open one or more day by day to observe the changing embryos.^ Since the 



^Place eggs under a sitting hen, or else in an incubator at 103° F. Eggs should be turned 

 each day. The incubation period is 21 days. A convenient way to plan for the study of 

 chick embryos is to place one or more fresh fertilized eggs in the incubator each day for 21 

 successive days, dating each egg, and then to open all eggs at once for study. To open eggs, 

 insert fine-pointed scissors through the shell and membrane and cut out a circular portion. 



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