31 8 L. V. HEILBRUNN. 



visible under higher power. It is about one micron in thickness. 

 The vitelline membrane must be thought of as the plasma mem- 

 brane of the egg. As I have pointed out before, students of 

 cellular mechanics have been blind to the fact that egg cells 

 are provided with visible membranes which are plasma-mem- 

 branes. They have often insisted that no one has ever seen a 

 plasma-membrane. It is easy to prove that the vitelline mem- 

 membrane of the Cumingia egg governs osmotic intercourse and 

 is therefore a plasma-membrane. 



When Cumingia eggs are put into hypertonic solutions they 

 shrink only very slightly. The stiffness of the vitelline plasma- 

 membrane prevents a marked shrinkage. Moreover, a weakly 

 hypertonic solution produces just as much shrinkage as a strong 

 one provided that it does not alter the membrane. On the other 

 hand if the hypertonic solution makes the membrane less rigid 

 by causing it to swell, then the egg shrinks to a great extent. 

 Facts such as these can be interpreted only on the assumption of 

 a stiff plasma-membrane. That this is the vitelline membrane 

 is certain, for there is no other membrane in the vicinity. Cer- 

 tainly there is none inside the vitelline membrane, for when the 

 egg has been left for some time in a hypertonic solution of con- 

 siderable strength then the coagulated cytoplasm shrinks away 

 from the vitelline membrane and presents a rough uneven sur- 

 face. Under these conditions it is obviously not surrounded by 

 any membrane. 



As to the chemical nature of the plasma-membrane it is 

 essentially protein. It swells in dilute acids or alkalies, and in 

 sodium chloride or sodium iodide solutions. Moreover it does 

 not contain any large admixture of lipoid as can be shown by test- 

 ing it with Scharlach R solution. 



Like many other marine eggs the Cumingia egg is surrounded 

 by a diffuse jelly of the same refractive index as sea-water and 

 therefore invisible. It can easily be demonstrated by various 

 vital stains (e.g., Nile blue sulphate) or by India ink suspensions. 

 This jelly has no apparent effect on the early developmental 

 phenomena. If the eggs are shaken a few times in a test-tube, 

 they are deprived of their jelly. Such eggs react in the same 

 way as those with jelly intact. 



