150 L. V. HEILBRUNN. 



are some of the questions which must be considered if our theories 

 are to be more than mere generalizations. 



The first point to be decided upon is the physical nature of 

 the egg contents. Embryologists in the past have often ob- 

 served flowings in egg cytoplasm and such observations indicate 

 a fluid composition. This would accord with Rhumbler's 

 demonstration of fluidity in other types of protoplasm. We 

 might accept these results without further comment if it were 

 not for the fact that recently there has been a tendency to regard 

 protoplasm as a gel. Kite ('13) has in fact concluded that the 

 protoplasm of the starfish egg is of this nature. The protoplasm 

 of the normal unfertilized sea-urchin egg is undoubtedly in a 

 typically fluid condition. If pressure is applied to eggs under- 

 neath a coverslip, the egg contents flow out, indeed, if the pres- 

 sure is vigorous enough, the protoplasm is shot out in a long 

 stream as from a pipette. Facts such as these are probably 

 known to many embryologists, similar observations have been 

 especially described by Reinke ('95) and Albrecht ('98). 



If the egg is essentially a fluid mass, what is there to prevent 

 it from diffusing through the sea-water? Two possibilities 

 exist, either (i) The substance of the egg is as a whole insoluble 

 in sea-water or (2) it is surrounded by a membrane insoluble 

 in sea-water. 1 The first possibility is excluded by the fact that 

 we know protoplasm to be in aqueous solution. We must there- 

 fore conclude that the egg is surrounded by a membrane insoluble 

 in sea-water. Careful observation reveals the existence of a 

 membrane around the unfertilized egg; just outside of the darker 

 substance of the egg cytoplasm, a dim outer line can usually be 

 made out. The faintness with which the outer margin of this 

 vitelline membrane appears is apparently due to the fact that 

 its refractive index is very close to that of sea-water. That the 

 appearance of a membrane is not the result of a diffraction 

 illusion is shown by the fact that the membrane may be isolated 

 by pressing out the egg contents, as Herbst first found. At 

 fertilization, as will be pointed out more fully later, it is this 



1 The oft-made assumption of a surface film like that found at the surface of 

 peptone solutions, etc., is really a special case of the first alternative. For such a 

 film could only exist at a surface of discontinuity, and this could only occur at 

 the junction of 2 immiscible fluids. 



