THE BIOLOGY OF THE CELL SURFACE 



posed of two curved rods joined together by a delicate but 

 clearly revealed bridge. 



Once the spermatozoon together with the middle-piece 

 is within the egg — the tail does not enter — it rotates through 

 } an angle of i8o degrees. This rotation may, however, 

 take place while the spermatozoon is still In the entrance- 

 cone. At this stage occur the following changes: the 

 sperm-head may reveal a more sharply stained outline, thus 

 giving the appearance of a hollow tube; the outline of the 

 middle-piece is no longer distinguished; and the middle- 

 piece granule is closely stuck to the base of the sperm-head. 



After rotation, the sperm-nucleus is directed toward the 

 centre of the egg. The middle-piece granule, hitherto a 

 closely fitting cap over the base of the sperm-head, slips off 

 carrying with it a thread of sperm-substance. Or It may 

 be that the sperm-head swells except In the region at which 

 the middle-piece granule is clamped to Its base. Figure 

 29^ shows a sperm-head at this stage. Certainly the 

 sperm-head now shows well defined difference in volume 

 and form; these changes take place with the separation of 

 the middle-piece granule.^ 



The sperm-aster, never found before the stage at which 

 the middle-piece granule draws away from the sperm-head, 

 Is now well defined and distinct. Its spherical central 

 portion, the astrosphere, appears in fixed sections as clear, 

 homogeneous substance which does not stain. The astral 

 rays are paths of clear granule-free ground-substance lying 

 between rows of the cytoplasmic constituents — mitochon- 

 dria, yolk spheres and oil drops. The astral configuration 

 noted in properly fixed eggs bears the closest resemblance 

 to that observed in the living egg, 



^ These changes in the sperm-head are not unlike those found in 

 free-swimming spermatozoa fixed after agglutination with specific 

 egg-sea-water that will later be discussed. 



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