CHAPTER TWENTY-THREE. 

 GROWING UP. 



1. In the Simplest Forms. Thus far we have spoken 

 about the process of reproduction and the work of the 

 parents. There is another very interesting side to the 

 matter. This is the growing up or development of the 

 young individual into the form of the parent. 



In the simplest forms this has been seen to be a rather 

 easy thing. In the case of bacteria, or Paramecium, when 

 the parent divides, each of the progeny has to double its 

 size. In the bacteria this requires only a half-hour or so 

 under favorable conditions. In Paramecium it takes a 

 little longer. Where the parent differs from the offspring 

 only in size the problem is merely one of growth. 



2. How About the Higher Organisms? When we come 

 to a higher plant or animal, in which there are numerous 

 different kinds of organs in the adult, we see that the 

 work of the embryo in growing up is very much 

 complicated. The one-celled fertilized egg has in it 

 absolutely none of the leaves and roots, or brain and 

 muscles and bones which the parent organism has. The 

 most marvelous thing of all to the biologist is that a cell 

 with nothing of any of these tissues or organs in it, with 

 only a little mass of protoplasm, can develop these 

 complex structures. The wisest biologist has no final 

 answer to this question, and yet every egg that develops 

 answers the question in its own practical way. Of course 

 the student sees that it is no answer to the question to say 



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