THE PROBLEMS OF DEVELOPMENT 



11 



regenerate the whole animal. What stimulates these cells in adults to develop 

 into new structures? Why do some animals, notably man, fail to regenerate 

 structures ? There is no obvious theoretical reason for this loss of regenerative 

 ability in man. The structures in man are no more complicated than in sala- 

 manders. The problem of obtaining regeneration in man seems to be a matter 

 of finding the conditions for regeneration in salamanders and applying these 

 conditions to man. 



Then there are the problems arising during the growth process, growth 

 being considered as a synthesis of new protoplasm. While development 

 is going on, growth goes on simultaneously. What regulates growth ? What 

 are the relative roles of food, age, heredity, etc., in the growth process? Tissue 

 culture shows that some cells divide indefinitely as long as they are nourished. 

 Why do cells in the body fail to go on and grow indefinitely? Growth usually 

 starts out slowly, then proceeds very rapidly, and finally slows down to a 

 very low rate. Figure 6 shows a hypothetical growth curve. One question 

 which faces us is whether cells can be rejuvenated after the aging process. 



The growth of the mammalian embryo offers some special problems, 

 since the embryo must derive its raw food materials from the blood of the 

 mother. The placenta establishes a pathway for the transfer of food from 

 mother to embryo. The relationship between developing egg and the mother 

 becomes complicated, and hormones provide and maintain this relationship. 



In the adult the development cycle is completed with the formation of 

 the sperm and egg for the next generation. At this point we ask how sperm 



Fig. 6. A graph of growth pat- 

 terned after the growth of micro- 

 organisms in test-tube cultures. 

 When a few cells are added to a 

 fresh culture medium they show a 

 latent period — i.e., a period during 

 which little growth takes place. 

 Then the number of cells increases 

 rapidly with time (increasing rate).' 

 Then growth may go along at a con- 

 stant rate for a while but eventually 

 it begins to slow down (decreasing 

 rate) and finally no growth occurs. 

 Growth curves of higher organisms 

 may deviate widely from this hypo- 

 thetical curve. 



TIME UNITS 



