PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 277 



can be made to flow more rapidly by placing a needle in the 

 pseudopodium just behind the advancing tip and then moving 

 the needle forward. When the pseudopodium comes to a stand- 

 still, forward movement of the needle tears through the surface 

 layer; the former fluid membrane has become a firm one. Pro- 

 nounced changes take place in degree of delicacy, consistency, and 

 tension of the plasma membrane coincident with physiological 

 and structural changes in the cefl. Division involves change in 

 the protoplasmic surface. Especially evident is this in the case 

 of the heavy pellicle of a protozoan, but equally true is it of all 

 cells, in particular those which divide by the process of fission, or 

 pinching in two. 



Abnormal surface changes occur in cells. The sea-urchin egg is 

 a quiet, spherical droplet of protoplasm. It may, on occasion, 

 assume amoeboid activity when pseudopodia are put out from 

 its surface, as in Amoeba. The normally smooth contour of the 

 cup-shaped, red blood corpuscles may form surface protrusions, 

 and the unripe sand-dollar egg may, in the presence of sperm, 

 send forth extraordinary protoplasmic processes (Fig. 175). 

 (These latter may be distortions of the surface layer or inner 

 protoplasm oozing out through the membrane.) 



Attempts have been made to determine the thickness of the 

 protoplasmic membrane. Remington estimates 100 A. U. for the 

 surface layer of the cell of the beet. Such figures apply to cells 

 that do not change their form, as is true of those belonging to 

 average plant tissue. The thickness of the membrane of an 

 active amoeba changes greatly. Difficulty in determining not 

 only the thickness but the actual presence of a protoplasmic 

 membrane is increased by the fact .that there is not usually a very 

 sharp boundary at the inner surface of the membrane. The 

 membrane often appears to grade imperceptibly into the inner 

 protoplasm. 



The Membranes on Cell Inclusions. — What is true of the 

 surface of the cell is true of most cell inclusions. The presence 

 of a nuclear membrane can be demonstrated in a convincing way 

 after the nucleus coagulates, for the membrane is then capable of 

 isolation as a stiff veil (Fig. 48). It is possible that this mem- 

 brane is a post-mortem product, especially in the light of the 

 behavior of some nuclei, which Chambers has shown may be 

 severed while in the cell and the halves fuse on coming into con- 



