THE MORPHOLOGY OF FERTILIZATION 5 



spermatozoa of Cerebratulus [lacteus Verrill ?) pay no attention to the 

 so-called micropyle present in the membrane round the eggs of this 

 nemertine, and can reach and fertilize the egg at any point on the 

 surface. The unfertilized egg of Megathura crenulata (Sowerby) is 

 surrounded by a tough membrane which is distinct from the 

 surface of the egg proper, and the same applies to human eggs, in 

 which the enveloping membrane is called the zona pelhicida. 

 Spermatozoa get through these barriers with the help of enzymes 

 located in their heads. Finally, most mammalian eggs are sur- 

 rounded by follicle cells, in two layers; the innermost of these con- 

 sists of densely packed, radially arranged cells and is known as the 

 corona radiata. Outside this there is a layer of sparsely distributed 

 cells, the cumulus oophonis. The enzyme hyaluronidase, contained 

 in or on the surfaces of most mammalian spermatozoa, assists in 

 the dissolution or depolymerisation of the intercellular cement, 

 hyaluronic acid, by which the follicle cells are stuck to the un- 

 fertilized egg surface. 



Cortical change. Having got through these 'barriers', the sper- 

 matozoon becomes attached to the surface of the unfertilized egg. 

 After attachment, the sperm tail may continue to move quite 

 vigorously, though in other cases, it sticks out from the egg sur- 

 face, motionless. The first visible reaction of the egg to the attach- 

 ment of the fertilizing spermatozoon — spermatozoa quite often 

 become attached to eggs but fail to fertilize them — is a change in 

 cortical structure, which, starting at the point of sperm attachment, 

 passes completely over the egg surface. The time relationships of 

 this reaction are discussed in chapter 9, Polyspermy. According to 

 J. C. Dan (1950^, p. 402), this change in cortical structure is 'a 

 visible wave which travels around the egg at speeds varying with 

 the species, . . .'. 'In the relatively fluid eggs oi Mespilia [globulus 

 (Linn.)) the passage of this wave is especially striking; it causes a 

 slight deformation of the surface layers of the egg, which gives the 

 impression that some sort of tension is being progressively re- 

 leased, or that a local band of contraction and expansion is passing 

 around the egg.' This wave of so-called contraction has been 

 observed by numerous students of fertilization, but it is doubtful 

 whether the word 'contraction' is apposite or even desirable, except 

 in special cases such as that of the brook lamprey, Entosphenus 

 lamot t enii (Lesueur), which does contract after fertilization (Okkel- 

 berg, I9i4)> or in the case of mammalian eggs. The German word 



