Plate II. 

 In Figs. 17, IS and 21 only a portion of tUe tail is represented. 



17. Young spermatozoon. Smear preparation. X 2000. 



18. Young spermatozoon in whicli the middle piece lias been formed. Axis 

 tlirough the middle piece. Smear preparation, x 2000. 



19. Young spermatozoon with a long anterior process. The centriole 

 can distinctly be seen in the middle piece. Smear preparation (well ex- 

 tracted). X 2000. 



20. Full grown spermatozoon. Smear preparation. X 80(5. 



21. Full grown spermatozoon more highly enlarged. Smear preparation 

 (well extracted). X 2000. 



22. Spermatozoon in ,the egg, already rotated. Thickening of the tail. 

 X 2000. 



23. Spermatozoon with rays appearing precociously around the middle 

 piece. The rays are centering to the centriole. x 2000. 



24. Spermatozoon with rays precociously developed. In this case the 

 centriole has abnormally escaped from the middle piece, x 2000. 



25. Spermatozoon in which an indentation has formed to mark the boundary 

 between the head proper and the anterior process, x 2000. 



26. Spermatozoon with a rounded head proper and a long anterior process. 

 X 2000. 



27. Spermatozoon with a rhomboidal middle piece. X 2000. 



28. The same preparation further- extracted. The middle piece has lost 

 its outline, and the centriole has been brought out distinctly. Non-fibrous 

 rays around the centriole should be noticed, x 2000. 



29. Spermatozoon, in which the middle piece has become vesicular. 

 X 2000. 



30. Spermatozoon with middle piece a little more swollen, x 2(X)0. 



31. Spermatozoon with the first indication of rays. Notice a knob at 

 the end of the tail. X 2000. 



32. Middle piece with a pentagonal outline. X 2000. 



33. The same preparation further extracted. The middle piece has lost' 

 its outline, the centriole alone remaining as a distinct body. Elongation 

 of the tail, x 2000. 



34. Middle piece surrounded by long rays. X 2000. 



35. Centriole with ray-system escaped from the middle piece vesicle. 

 X 2000. 



36. Centriole just after its division. Tlie middle piece vesicle is about 

 to disappear, x 2000. 



37. Sperm asters with new ray systems. No centrosome is present. X 2000. 



38. Sperm asters with a synthetic spindle between them. X 2000. 



39. Sperm aster with much centroplasm. X 20(X). 



40. Sperm aster more advanced. Two centrioles in a degenerating centro- 

 some. The rays are about to disintegrate (only a portion of the rays are 

 represented), x 2000. 



