FERTILISATION 



47 



(i) Activation 



By 'activation' we mean the setting in train of a series of changes which 

 bring the egg out of the quiescent state in which it awaited the arrival of a 

 spermatozoan and start it off on the course of development. These changes 

 take somewhat different forms in different groups, but there are certain 

 common elements which are nearly always found. First in point of time, 

 there may occur, in eggs surrounded by thick jelly, a reaction of the egg 

 surface which assists the sperm in penetrating these outer coverings. For 

 instance, in some echinoderm eggs, a conical projection pushes out from 

 the egg surface and, as it were, catches hold of a sperm and draws it 

 inwards through the jelly. Such happenings are, however, not found in 



all eggs. 



Some kind of surface reaction of the egg is nearly always produced by 

 the sperm. The most important and widespread form of the reaction is a 

 change by which the first sperm which penetrates renders the egg surface 

 impenetrable to later sperm. The exact nature of this change is still un- 

 known; it may even differ in different groups. In many of the naked 

 marine eggs, it is made visible by the formation at the surface of the egg 

 of a new membrane, the 'fertilisation membrane', which hfts a little way 

 off the egg immediately after activation (Runnstrom, 1952^, b). This is 

 very well seen in echinoderms; and in them it appears to be formed by 

 the swelling and breaking up of a thin layer of colourless granules which 

 can be found just below the surface of the ripe, unfertilised egg (Fig. 3.2). 

 It seems, however, that in some species of echinoderms if not in all, the 



Fertllizatlo" membrane ^i 

 ^ vlt.m. 





Yolk (endoplasi 



egz 

 -p.s.f. 

 Perivitslline ipace 



Unfertilized egg 



Fertilized egg 



Figure 3.2 



Diagram showing the elevation of the fertiHsation membrane in the sea- 

 urchin egg. In the unfertiUsed egg there is an inner layer of pigment granules 

 [pig. gr.) and an outer layer of cortical granules ('Janus Green granules', 

 JGG). The perivitelline space appears between these two layers. (From 

 Runnstrom 1952, after Motomura.) 



