2 THE BREEDING OF ANIMALS 



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mind are hidden in the ganglion cell." It is now clearly 

 apparent that the great questions of reproduction, in- 

 heritance and development, involving as they do the 

 problems of embryology and evolution, are intricately 

 bound up with the structure and functions of the cell. 



2. The germ-cells. The heritage of the species is 

 contained within the germ-cell. The microscopic egg of 

 the female carries within its minute structure the germ 

 characters of all the maternal ancestors. The germ-cell 

 of the male, the spermatozoon, holds within its exceed- 

 ingly minute compass the sum total of all the heritable 

 characteristics of the paternal ancestors. All cells are 

 derived from other cells. Virchow's claim made in 1855 

 that every cell must have been formed by cell division 

 from some previously formed cell, has now been definitely 

 established. Not only does growth and development 

 take place by cell division in the fertilized egg-cell, but 

 the egg-cell itself is directly derived by cell division from 

 an egg-cell of the immediately preceding generation, and 

 so on indefinitely. 



3. The cell. The cell is a mass of protoplasm con- 

 taining a nucleus, and both nucleus and protoplasm arise 

 through division of the corresponding elements of a pre- 

 existing cell. 1 



The word cell is derived from the Greek and means a 

 hollow chamber. The term came into common use be- 

 fore the form and structure of the cell were well under- 

 stood. The cell-wall which is characteristic of most 

 cells is not an essential part of its, structure. It is also 



1 Wilson, "The Cell in Development and Inheritance," p. 19, 

 1906. 



Ley dig, " Lehrbuch der Histologie," p. 9, 1857. 

 Schultze, " Arch. Anat. u. Phys.," p. 11, 1861. 



