THE BIOLOGY OF THE CELL SURFACE 



later stage, depending upon the species of egg. As we shall 

 see, this old problem of differentiation is still the major one 

 of the study of development. 



With successive cell-divisions during cleavage, the 

 blastomeres become progressively smaller. If this process 

 were to continue indefinitely, the size of the cells would 

 approximate zero and cell-division would come to a stand- 

 still. But this never happens, since in later stages the 

 cells transform food into protoplasm. In many cases the 

 t^g soon after hatching as a larva takes in food; or 

 the embryo, as that of the chick, utilizes the yolk which 

 is present as reserve food material within the egg. 



Generally, growth is increase in size. A cell may grow 

 as such by the intake and transformation of food. In the 

 initial stages of the development of an egg, cell-multiplica- 

 tion procedes without any increase of volume. The total 

 mass of the egg of a sea-urchin, for example, at the end of 

 its cleavage-period is approximately the same as at the 

 beginning. In the larval stage, cell-multiplication follows 

 upon the intake and transformation of food by the cells; 

 thus here through the combined process of increase in vol- 

 ume by the single cells and increase in their number, growth 

 of the embryo is attained. 



In the adult organism derived from the egg, cell-division 

 operates daily in the restitution process. Cells of the 

 human skin are constantly being replaced. Each day the 

 blood-forming tissues in bones must throw into the blood- 

 stream millions of red blood corpuscles to compensate for 

 the disintegration of older ones. Where there is no more 

 capacity for cell-division, any injury becomes tragic; thus 

 the capacity for regeneration in the human central nerve 

 system is meagre and the possibilities of repair after surgery 

 slight because its cells have lost the power of division. 

 Organs which possess reparative capacity have retained 

 the capacity for cell-division. 



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