REPRODUCTION AND EARLY DEVELOPMENT. 109 



the carriers of all the hereditary traits that may be trans- 

 mitted. The protoplasms of these reproductive cells 

 have crowded into them all the potencies of the bone 

 and muscle and nerve cell; of all the organs described in 

 the preceding chapter, and of all the instincts and powers 

 of the mature member of the species. Heredity gives 

 continuity to the successive generations of animals; 

 makes it possible for us to speak of a species or race of 

 animals. 



It does not explain a quality to say that it is inherited, 

 but it is often convenient to distinguish between charac- 

 teristics that come directly from the parents and those 

 that come through the environment. 



117. Library Exercises. Would the influence of the parent on 

 the embryo after the union of ovum and sperm be a matter of he- 

 redity or of environment? Why? Do both the ovum and the sperm 

 contribute to the hereditary qualities? Do they contribute equally 

 in the long run? Evidences? Are differences between offspring 

 due only to environment? Or might offspring inherit any of the 

 differences which exist between them and their parents and among 

 themselves? What is atavism? 



118. Summary. 



1. Adult individuals grow old and die, because the 

 cells of which they are composed gradually lose their 

 powers. 



2. At certain stages of life individuals have the power 

 to separate from themselves, in one w r ay or another, single 

 cells or groups of cells, which are young again and have 

 the power of independent and separate life. This is 

 reproduction, and makes possible the continuance and 

 even increase of the species, in spite of death. 



3. There are several methods which animals use to 

 accomplish the result : 



