142 MORPHOLOGY OF FERTILIZATION 



sidered the possibility that either form might be the precursor 

 of the other. In one type of spennatozoon (Fig. 19), fomid in 

 both hving and fixed preparations, the acrosomal region appeared 

 as a short stalk terminating in an apical knob. This is the type of 

 spermatozoon illustrated by Vasseur ( 1947 ) in darkfield studies 

 and, apparently, by Afzelius ( 1955 ) in electron microscope studies 

 of thin sections (Figs. 14e and 15B). The other type of sperma- 

 tozoon (Fig. 20), found only in fixed preparations, has a pro- 

 foundly different appearance. Extending from the apex of the 

 sperm head is a slender filament which is much longer than the 

 acrosomal region of the first type. There is no terminal knob, the 

 middle piece lies in a postero-lateral position, and the flagellum 

 and acrosome filament lie nearly at right angles to each other. All 

 these characteristics are typical of the reacted sea urchin spenna- 

 tozoon as shown by Dan. It seems probable that Fig. 20 represents 

 a reacted spermatozoon. Tyler's (1952) electron microscope stud- 

 ies of fertilizin-treated sea urchin spermatozoa would seem to 

 support this view. After reversal of agglutination, these sper- 

 matozoa too showed the middle piece postero-laterally displaced 

 from its normal location at the base of the head. Since Rothschild 

 and Tyler found spermatozoa with long acrosomal filaments only 

 in -fixed preparations, the possibility exists that the fixative itself 

 or some feature of the process en route to fixation may have 

 stimulated the acrosomal regions to produce these filaments ( vide 

 supra, living i;.s fixed spermatozoa of Nereis; for a more detailed 

 discussion see Colwin and Colwin, 1956 ) . 



Staiiishes. A more striking example of the acrosome reaction 

 with formation of an acrosome filament was found in several 

 starfish species by Dan ( 1954 ) . When sperm suspensions were 

 made in sea water plus egg albumin ( Metz, 1945 ) to which was 

 then added homologous egg water, irreversible agglutination 

 occurred. Both phase contrast (Fig. 21) and electron microscope 

 studies (Fig. 23) showed that within such preparations there 

 were spermatozoa with long slender acrosome filaments of con- 

 siderable rigidity. The filament extended from the apex of the 

 acrosomal region for about 25 microns. In the reacted sperma- 

 tozoon (Fig. 22A) the middle piece was more nearly spherical 



