22 PHYSIOLOGICAL TRIGGERS 



agent to penetrate to a subsurface site of action. This site must, then, be differ- 

 ent from and presumably 'internal' to the surface site of activation in conju- 

 gation. Similarly artificial parthenogenetic agents and the spermatozoa may 

 initiate activation of the metazoan egg through different routes or receptors. 

 In fact different parthenogenetic agents may activate the egg by action at 

 different sites in a chain of reactions. Indeed each of several parthenogenetic 

 agents may be specific for a particular site in such a chain (62, 67, 97). 



According to the account presented above, the mating type substances play 

 a primary role in the fertilization of Paramecium. These substances are proteins 

 or closely associated with proteins (62, 65, 68). Although they are not antigenic, 

 they appear to be closely associated with antigenic material (68). They are 

 located on the cilia and possibly other parts of these ciliates. They probably 

 interact in antigen-antibody-like fashion, they are responsible for the primary 

 specificity of conjugation and for the initial adhesion or mating reaction. Fi- 

 nally, their interaction initiates or 'triggers' the subsequent events of conju- 

 gation. These relations are summarized in figure 3. 



TRIGGER ACTION ON THE SPERMATOZOAN 



Until recently the normal, motile spermatozoan was not generally considered 

 to require any special preparation in anticipation of its union with and acti- 

 vation of the egg. Lillie (48), however, did suggest that the spermatozoan 

 itself must be 'fertilized' before it can activate the egg. Evidence in support 

 of such a view has come from recent studies of J. C. Dan, and these have been 

 confirmed in part by Colwin and Colwin and by Metz and jNIorrill. 



Examination of spermatozoa under high magnification, preferably at the 

 electron microscope level, reveals that the morphology of the sperm acrosome 

 region may vary. The variation ranges from acrosomes of compact to others 

 of filamentous form. These extremes are the usual ones but intermediate con- 

 ditions are sometimes found. Dan (22) considers the 'compact' condition of the 

 acrosome the normal or original and the filamentous form a product of the 

 normal or compact form. Conversion of the compact to the filamentous form 

 constitutes the 'acrosome reaction.' 



In the sea urchin (22) the acrosome reaction takes the form of a breakdown 

 of the membrane surrounding the acrosome. Calcium ion is required for this 

 reaction in the sea urchin (24). In the starfish (20, 23, 71) the reaction involves 

 the appearance of a very long {ca. 25 ^u), fine filament from the acrosome region 

 (fig. 4). Dan suggests that this filament is discharged from the acrosome region. 

 Very long filaments have also been observed in mollusc (25, 94), echinoid (94), 

 holothurian (19) and enteropneust (18) sperm. In the annelid Nereis (71) the 

 prominent cone-shaped acrosome may be replaced by a fine filament, possibly 

 as a result of lysis or retraction of superficial acrosome material (fig. 5). Sabel- 

 laria sperms produce filaments of varying length (16). 



