GROWTH 341 



zoan individual is in one sense a colony of cells in which there 

 is unity of action. But in order to secure the best results, the 

 division of labor means differentiation, and differentiation 

 carried very far means loss of power of recuperation and of cell 

 division. Thus the functions of the somatic cells contribute 

 to the perpetuation of the race by fostering the gametes and 

 further nursing the embryo in many cases until it is well ad- 

 vanced in development. 



726. Regeneration. — That the loss of the power of recupera- 

 tion goes hand in hand with differentiation, appears from a 

 study of the phenomena of regeneration. An egg has the power 

 of regeneration like that of a protozoan, so that from a fragment 

 of an egg a complete embryo may develop. This is true even 

 of Vertebrate eggs. Among the lower invertebrates such power 

 of regeneration exists even in the adult. An anemone or star 

 fish may be cut in two, and the parts will regenerate all the 

 organs that were removed. This is not possible with a crab or 

 crayfish. But even the Crustacean will regenerate an append- 

 age that was broken off. So as we pass to higher forms the 

 power regularly decreases. In young Amphibia, appendages 

 may still be regenerated, but in Birds and Mammals the power 

 extends only to the healing of wounds, which is also a regenera- 

 tion process. 



727. Mechanics of Growth.— Differentiation begins at a very 

 early stage. Even in the lancelet the entoderm is distin- 

 guishable from the ectoderm before gastrulation takes place, 

 and the cells of the medullary plate are distinguishable from 

 the rest of the ectoderm before the medullary groove is formed. 

 But it will not be necessary to describe the individual types of 

 tissues here, since this was done in connection with the anatom- 

 ical description of types. There are, however, some points 

 about the mechanics of growth which should be noted. A 

 naked protoplasmic cell like Amoeba expands freely with 

 growth and often when there is a cell membrane it is elastic 



