54 COMPARATIVE PHYSIOLOGY. 



ment of the egg may take place independently of the influence 

 of the male sex. The best examples of parthenogenesis are to 

 be found among insects and crustaceans. 



It is to be remembered that, while the cells which form the 

 tissues of the body of an animal have become specialized to 

 discharge one particular function, they have not wholly lost 

 all others ; they do not remain characteristic aniceboids, as we 

 may term cells closely resembling Amoeba in behavior, nor do 

 they wholly forsake their ancestral habits. They all retain the 

 power of reproduction by division, especially when young and 

 most vigorous ; for tissues grow chiefly by the production of 

 Vj new cells rather than the enlargement of already mature ones. 

 Cells wear out and must be replaced, which is effected by the 

 processes already described for Amoeba and similar forms. 

 Moreover, there is retained in the blood of animals an army of 

 cells, triie amceboids, ever ready to hasten to repair tissues lost 

 by injury. These are true remnants of an embryonic condition ; 

 for at one period all the cells of the organism were of this un- 

 differentiated, plastic character. But the cell (ovum) from 

 which the individual in its entirety and with all its complexity 

 arises mostly by the union with another cell (sperm,atozodn), 

 must be considered as one that has remained unspecialized 

 and retained, and perhaps increased its reproductive functions. 

 They certainly have become more complex. The germ-cell 

 may be considered unspecialized as regards other functions, but 

 highly specialized in the one direction of exceedingly great ca- 

 pacity for growth and complex division, if we take into account 

 the whole chain of results ; though in considering this it must 

 be borne in mind that after a certain stage of division each 

 individual cell repeats its ancestral history again ; that is to 

 say, it divides and gives rise to cells which progress in turn as 

 well as multiply. From another point of view the ovum is a 

 marvelous storehouse of energy, latent or potential, of course, 

 but under proper conditions liberated in varied and unexpected 

 forms of force. It is a sort of reservoir of biological energy 

 in the most concentrated form, the liberation of which in sim- 

 pler forms gives rise to that complicated chain of events which 

 is termed by the biologist development, but which may be ex- 

 pressed by the physiologist as the transformation of potential 

 into kinetic energy, or the energy of motion. Viewed chemi- 

 cally, it is the oft-repeated story of the production of forms, of 

 greater stability and simplicity, from more unstable and com- 



