502 BULLETIN OF THE 



tion of the yolk results in the formation of the protoplasmic filaments 

 and the "Leitband"; the latter, however, is not constant, nor does it, 

 when present, always serve to guide a spermatazobn ; on the contrary, 

 a spermatozoon may penetrate the membrane at any point of the 

 watch-glass, even near its margin. The term " Axenstrang " is substi- 

 tuted for " Leitband," as the function implied in the latter word can- 

 not be proved to exist. The foremost or "preferred spermatozoon" 

 enters the yolk in toto, not leaving any part of its tail behind, as Cal- 

 berla claims ; but as soon as the head has entered the egg membrane 

 the activity of the spermatozoon seems to cease, the tail becomes stiff, 

 and the whole is drawn onward. The head part becomes more and 

 more elongated, as it approaches the yolk. All other spermatozoa which 

 penetrate the " watch-glass " a greater or less distance do not thus cease 

 their activity, and in addition to the motion of the tail there seems to be 

 an amoeboid change in the outline of the head ; a wave-like motion is 

 observed to pass forward along the head to its free end. If the head 

 has advanced near to the inner surface of the membrane, it sends for- 

 ward a fine filament, like a pseudopod, which traverses the interval ; 

 waves advance along this filament, which becomes swollen at the end, 

 and finally the swollen part becomes detached as a clear vesicle in the 

 perivitelline space. 



The retraction of the yolk is due to an actual contraction of its sub- 

 stance rather than to an invasion of water as claimed by Calberla. The 

 contraction is of such a nature that at one time the half of the yolk occu- 

 pying the active end of the egg has the form of a truncate right cone, 

 while the other half retains its ellipsoidal outline. At this time, about 

 three minutes after the union of sperm and eggs, a mass of clear proto- 

 plasm (Calberla's Dottertropfen) rises up in the centre of the truncate 

 surface, enlarges and extends till, in about three minutes, it reaches the 

 inner surface of the membrane, which, by its amoeboid motion, " it licks 

 ©ff," and then with many changes of form it retires. By this process it 

 incorporates with it the vesicles which Calberla called " Randtropfen," 

 but which are not all of them the retracted ends of the protoplasmic 

 filaments left by the retiring yolk, for a part at least were formed by 

 the spermatozoa in the way above indicated. This "Dottertropfen," 

 occasionally at least, also envelops unaltered spermatozoa which have 

 effected their entrance into the perivitelline space. It is not simply the 

 again protruded " Leitband," for it is occasionally absent, and when 

 present is of greater volume than the latter. This protoplasmic projec- 

 tion exercises an active supplemental role in fecundation. While to one 



