THE BIOLOGY OF THE CELL SURFACE 



cificity. The phenomenon of specificity generally we relate 

 to chemical constitution and not to physical properties. 

 No valid reason can be proffered for assuming that spe- 

 cificity in fertilization is unlike that met with in other bio- 

 logical processes. Unless such reason is forthcoming, we 

 must assume specific fertilization as chemical. Where, as 

 in eggs of some species, a substance has been isolated upon 

 which fertilization depends, we are warranted again in 

 postulating that the fertilization-reaction is chemical — one 

 between this substance and the spermatozoon. Finally, 

 the changes, so strikingly visible in many eggs, whereby 

 the vitelline membrane is separated, can not, as we have 

 seen, be regarded as the initial reaction in fertilization: 

 membrane-separation, a physical process, though common 

 to all eggs, is the result of the fertilization-reaction and not 

 the reaction itself. 



What happens is this: the spermatozoon sets off a first 

 explosion in the narrow area of the egg-surface which it 

 touches, and kindles the spark which leads to a chain of 

 explosions. The fertilization-reaction is thus a trigger- 

 reaction. A large portion of the surface is shattered by the 

 explosive effect with gas-exchange and heat-liberation. 

 One can in many eggs see this break-down of the surface 

 by which the egg loses substance and the membrane is 

 separated. The pressure between egg and membrane, in 

 the perivitelline space, probably is considerable as the 

 ectoplasmic colloids disintegrate and go into solution. 

 Water rushes in and further distends the still ductile mem- 

 brane which then sets as a stiff structure. The fully 

 separated membrane then becomes brittle; one can more 

 easily remove it immediately after its separation than later 

 when it becomes tough. But these changes in the mem- 

 brane are due to its separation from the Qgg, for it no longer 

 forms part of the living system. What therefore are impor- 

 tant for the fertilization-reaction are the underlying surface- 



20.p 



