28 PHYSIOLOGICAL TRIGGERS 



induced by a specific agent from eggs and that this triggering is a necessary 

 preHminary to fertihzation of the egg by the spermalozoan. 



ASPECTS or EGG ACTIVATION 



The various aspects of fertihzation of the egg have been considered in detail 

 in a number of recent reviews (15, 62, 85, 86, 97, 99, 115, 116, 119). These 

 should be consulted by readers interested in a complete coverage of the subject, 

 for only two aspects of the fertilization of the egg need be considered here. 

 These are the activation initiating mechanism of fertilization and the nature 

 of the propagated responses of the egg upon fertilization. 



a) Activation Initiating Mechanism. Although the activation of the egg 

 has been the subject of investigation for many years, the mechanism of this 

 reaction remains obscure. From morphological studies it is clear that the re- 

 action requires only superficial union between the egg and the activating 

 sperm. This is especially evident in Nereis (47), the starfish (14, 28, 40), Urechis 

 (79, III) and Saccoglossus (18), for in these forms the egg undergoes definite 

 fertilization changes before the sperm nucleus is drawn into the egg cytoplasm. 

 Furthermore, the activating sperm may be removed from the egg surface by 

 microdissection (30) or by natural means in hybrid crosses (9, 51, 97). This 

 clearly indicates that the sperm-egg union required for the activation reaction 

 is not a very firm union. Finally, the area of contact would appear to be very 

 small. In the starfish and Saccoglossus at least the initial stages of activation 

 of the egg are apparently initiated by association of the sperm acrosome fila- 

 ment with the egg surface or cortex. If this association involves only contact 

 with the egg surface, then the area of contact for Asterias would appear to be 

 of the order of 0.002 /jr. This value is calculated from the diameter of the acro- 

 some filament near its tip (approximately 0.05 ju, assuming no flattening, in Fig. 

 4B). 



Many investigators have considered the activation initiating mechanism 

 to consist of an interaction of relatively specific egg and sperm substances. 

 This view was first proposed in detailed form in Lillie's (48) 'fertilizin theory 

 of fertilization.' The interaction of these substances is presumed to initiate a 

 chain of events which culminates in the morphological, physiological and 

 biochemical phenomena of egg activation just as interaction of the mating 

 type substances initiates activation in Paramecium (see first section). Attention 

 has been directed primarily to two substances, fertilizin from the egg and 

 antifertilizin from sperm. In the sea urchin, fertilizin constitutes the jelly 

 layer that surrounds the egg (113; fig. 6) and it or a similar agent may also 

 reside within the egg (77, 78) although this has been disputed (13). On standing, 

 the sea urchin egg jelly layer gradually dissolves and in so doing charges the 

 surrounding sea water with fertilizin. Antifertilizin is located on the sjierm 

 surface (116) but may be obtained in solution by heating (29), freeze thawing 

 (112) or acid extraction (127) of sperm. 



