160 L. V. HEILBRUNN. 



membrane appears as a thin membrane at some distance from 

 the egg surface, which is now bounded by the above-mentioned 

 hyaline layer. On the other hand, the swollen membrane 

 appears as a homogeneous layer surroundng the egg, a layer in 

 which neither the inner boundary of the vitelline membrane, nor 

 the outer boundary of the hyaline layer, make their appearance. 

 In addition to the purely morphological differences, there are 

 other distinguishing features. Among these may be mentioned 

 the fact that swollen membranes are always sticky, and as a 

 result, eggs with such membranes tend to agglutinate. Normal 

 elevated membranes are never sticky. Another criterion depends 

 on the fact that elevated membranes collapse when placed in a 

 solution of egg albumen (or other colloid). 1 Swollen membranes 

 are of course unable to collapse. 



B. Permeability Changes in the Vitelline Membrane. 



The elevated membrane is known to be readily permeable 

 to electrolytes. 2 Hence, since it offers considerable resistance 

 to their passage before elevation, it must undergo a change in 

 permeability at some stage in the process. An attempt was 

 made to determine if this increase in permeability took place 

 before or after membrane elevation. In the first case, it might 

 be considered as causally related to the process, and R. Lillie 

 has in fact suggested that the cause of membrane elevation is 

 an increased permeability of the "plasma-membrane." Ex- 

 periments, however, have shown that the increase in permea- 

 bility follows rather than precedes membrane elevation. Im- 

 mediately after elevation, the membrane is still more or less 

 impermeable to electrolytes. The following experiments show 

 this to be the case: 



August 21, 1913. Eggs from a single female were washed 

 twice and then gathered into about 10 c.c. of sea- water at the 

 bottom of a small beaker. Four or five drops of diluted sperm 

 were then added, and the beaker shaken. At intervals of 

 i, 2, 3, 4, 5 minutes after insemination, the eggs were re- 



1 It is only some few minutes after elevation that this collapse can be produced 

 by a colloid. Cf. p. 162. 



2 Cf. Loeb, "Artificial Parthenogenesis and Fertilization," p. 208. 



