THE BIOLOGY OF THE CELL SURFACE 



that cell-structure which controls ingress and egress is the 

 ectoplasm which in large part has chemically the same 

 make-up as the remainder of the cytoplasm since endo- 

 plasm and ectoplasm are one continuous, though differen- 

 tiated system. 



The question which disturbs the physiologists who say 

 that there must be a membrane in order to keep the cell- 

 contents from flowing out and becoming miscible with the 

 surrounding medium may be answered here. The cell- 

 contents withstand outflow because the cell material is 

 one continuous system. It is not like mercury which is 

 now one mass and then many drops which flow together 

 again; rather, protoplasm is a cohering though extremely 

 thin dilute jelly-like solution, whose biological character is 

 revealed by its strong regional differentiation. Excreted 

 or secreted material, including colloids, gets out of a cell 

 when it is no longer a living part of it. It is broken off and 

 dispelled. Instances in support of this statement are not 

 wanting. In egg cells we have seen that at fertilization the 

 colloids in the surface break down and escape through the 

 vitelline membrane. Material moves out of cells by 

 secretion — that is, the cell breaks off part of itself and 

 washes this part away. 



The largest question in the permeability problem relates 

 to the entrance of substances into the cell. If we attempt 

 to answer this question we do it on the basis that the cell- 

 surface is living ectoplasm, continuous with the remainder 

 of the protoplasm, and is made up of a brushwork of 

 filaments. 



In part, the question how substances enter cells is bound 

 up with another, namely, why some substances and not 

 others get into cells. For example, in the gut and in the 

 kidney salts or sugars when present in equimolecular con- 

 centration do not cross the cell-surface in the same amount. 

 These two cases constitute stock arguments of the vitalists 



142 



