168 



B. A. AFZELIUS 



Fig. I. The acrosomal region of a Psanvnechinus miliaris 

 spermatozoon that has undergone the acrosomal reaction 

 through treatment with egg water. Magnification 100,000. 



Fig. 2. The acrosomal region of an Echinus esciileiUiis 

 spermatozoon that has undergone the acrosomal reaction in 

 the immediate vicinity of an egg. Magnification 116,000. 



Fig. 3. The acrosomal region of a Strongyloceiitrotus droe- 

 bachiensis spermatozoon that has undergone the acrosomal 

 reaction with the acrosomal filament projecting from the 

 sperm nucleus to a process on the egg surface. The nuclear 

 indentation is obliquely cut. Magnification 96,000. 



Fig. 4. The acrosomal region of a Strotigylocentrotiis dioe- 

 bachiensis spermatozoon that has undergone the acrosomal 

 reaction and projects an acrosomal filament from the sperm 

 nucleus to the egg surface at the bottom of the picture. The 

 filament is broken into three fragments. Magnification 

 > 84,000. 



acrosomal globule and its attached segment of cell 

 membrane to separate from the sperm head. How- 

 ever, the filament, which in many cases observed had 

 elongated to a length exceeding 0.5 //, has retained 

 its connection with both the sperm nucleus and the 

 separated acrosomal region. Figures 3 and 4 illu- 

 strate this stage of the acrosomal reaction in Strongy- 

 locentrotus drocbachiensis spermatozoa. In figure 3 

 the filament and acrosomal ring are shown to advan- 

 tage while the nucleus is not sectioned in such a way 

 as to show its apical indentation. Figure 4 illustrates 

 the nuclear indentation. In both pictures the acro- 

 somal filament is fractured. It is not known whether 

 this is due to imperfect fixation or to mechanical 

 damage in subsequent treatment. 



Conclusions. — The two parts of the acrosomal 

 region found in sea urchin spermatozoa, viz., the 



acrosomal globule and the material within the inden- 

 tation in the nucleus, behave differently when the 

 spermatozoon comes in contact with the egg or is 

 treated with egg water. The acrosomal globule is 

 expelled from the cell but evidently does not dissolve 

 within the first 3 minutes at the temperature used 

 during the experiments (4-10 C). The material 

 within the nuclear indentation, on the other hand, 

 remains on the inner side of the cell membrane and 

 elongates to form the acrosomal filament which 

 coexists with the expelled acrosomal globule. 



The phenomenon of the acrosomal reaction was 

 first described by J. C. Dan (2) in sea urchin sperma- 

 tozoa and later her studies were extended to a 

 variety of other animal classes and to different cir- 

 cumstances evoking the reaction (3, 4, 5, 11). The 

 possibility to perform ultra-thin sectioning (accord- 



