FERTILIZATION 115 



tiny, whitish points in the nearly colorless fluid which 

 fills the pollen tube. 



The living sperm, as seen under the microscope, has 

 a very large nucleus, surrounded by a thin and almost 

 colorless sheath of protoplasm, which is somewhat 

 thicker at the forward end containing the spiral, cihated 

 band. 



The movements of the sperms are easily observed by 

 mounting the piece of the ovule containing the pollen 

 chamber with the pollen tubes, so that the enlarged ends 

 of the tubes are uppermost. The upturned ends of the 

 pollen tubes are so transparent that they scarcely 

 obscure the view. The cilia begin to move sluggishly 

 while the sperms are still fast together, and this move- 

 ment is accompanied by pulsating and amoeboid move- 

 ments which continue for an hour before the sperms 

 separate. After the separation they swim for half an 

 hour or more in the constantly enlarging body cell 

 before they escape into the main portion of the tube. 

 Occasionally the sperms are still attached to each other 

 after they have escaped into the general cavity of the 

 tube, and in such cases their movements are awkward, 

 because they naturally try to move in different direc- 

 tions. When they become free from each other the 

 principal movement is straight ahead, with a rotation 

 on the long axis. The sperms swim up and down the 

 tube, going up as far as the diameter of the tube will 

 permit and then coming back. The amoeboid move- 

 ments of both the protoplasm and the nucleus are quite 

 noticeable, especially while the sperm is changing its 

 direction. Most of the changes in direction occur when 

 the sperm bumps against the wall of the tube. At the 



