THE BIOLOGY OF THE CELL SURFACE 



able reagent which faithfully preserves the oil and yolk as 

 well as the nuclear structures, cutting them into thin sec- 

 tions (three or four microns thick) and coloring them with 

 a dye which stains both cytoplasmic inclusions and chroma- 

 tin material. 



In sections of eggs fixed after insemination, the following 

 facts can be ascertained: The ectoplasmic breakdown has 

 occurred. The germinal vesicle has broken down and the 

 first maturation spindle has formed. To the fertilization- 

 cone when fully formed the spermatozoon is fixed by the 

 sharp anterior spike, the perforatorium, which traverses the 

 perivitelline space whilst the head, middle-piece and tail 

 remain external to the egg. The fertilization-cone as in 

 the living egg projects beyond the egg-surface and later 

 recedes. 



What one learns in addition to these details, verifying 

 the observations on the living egg, concerns the change of 

 the perforatorium of the spermatozoon and of the cone. 

 As the spermatozoon enters deeper into the cone, further 

 granules appear at its tip, whilst it itself gains in staining 

 capacity. The cone is sharply marked off from the remain- 

 der of the egg-cytoplasm; it is homogeneous in appearance, 

 free from yolk and actually different in physical make-up. 



At about fifty minutes after insemination, the sperm- 

 head disappears within the egg. The cone, as can be recog- 

 nized by its staining and by its maintenance of form, acts as 

 a solid body which sinks into the tgg exerting tension upon 

 the sperm-head which stretches like a ductile strand. Fi- 

 nally the strand breaks at the surface of the egg so that the 

 external portion remains outside. This Is the middle-piece; 

 hence, only the sperm-nucleus enters the egg of Nereis. 

 Cone and attached sperm-head, acting as one complex, 

 now revolve through an angle of i8o degrees. Soon there- 

 after a star-shaped formation, the sperm-aster, with a 

 minute granule, the centrosome or centriole, at its centre, 



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