56 SPERMATOGENESIS AND FECUNDATION OF ZAMIA. 
can hardly be seen. The spermatozoids after starting motion soon. 
break loose from the stalk cell, which quickly collapses. Shortly after 
this they begin to gradually round up and pull apart, the ciliar motion 
continuing very active meanwhile. As they are located previous to — 
this, closely pressed together with their major axes crossing the tube 
(fig. 47a), there is not room for them to separate by pulling directly 
apart, so they stretch in opposite directions, bending toward the longi- 
tudinal axis of the tube (fig. 47 b and c). They continue to round up 
more and more and finally pull entirely apart (fig. 47d). In one tube 
of this nature, when first cut off and placed in sugar solution, stream- 
ing motion of the protoplasm of the pollen tube was noticed in some 
strands immediately above the spermatozoids. This was entirely 
interrupted as soon as the motion of the spermatozoids began. 
In the examination of fixed and stained sections of tubes in which 
the spermatozoids were swimming, there seems to be very little proto- 
plasm in the tube. The living tubes, however, present every evidence 
of being very turgid and completely filled, and such is doubtless the 
case. In tubes which are cut off quite long and are uninjured in the 
transter, such as the tube mentioned above in which streaming motion 
was observed, the entire tube is seen to be filled with granular proto- 
plasm, with definite strands occurring here and there. The sperma- 
tozoids when they first begin moving have some difficulty in breaking 
through the plasma membrane of the pollen tube cell and entering the 
general protoplasm of the pollen tube (see page 53); when this is 
accomplished, however, they swim back and forth with considerable 
ease. It is an interesting sight to see the two giant spermatozoids 
moving around vigorously in the pollen tube, bumping against each 
other and the wall of the tube in their reckless haste. They seldom 
escape from the upper cut end of the pollen tube, although they as fre- 
quently swim toward this end of the tube as the other end, so far as 
could be observed. In many cases the pollen tubes were cut so that the 
spermatozoids escaped into the solution, and in numerous other cases 
mature turgid tubes burst in the process of cutting, discharging the 
uninjured spermatozoids in the sugar solution. The writer was thus 
able in many cases to study the spermatozoids swimming free and 
observe their unobstructed motion. The plasma membrane of the sper- 
matozoids is very tender, however, and is commonly broken in attempt- 
ing to remove them from the pollen tube. Whenswimming free without 
pressure they are slightly ovate, nearly round or compressed spherical 
(fig. 52). They vary greatly in size, but are commonly slightly longer 
than broad, ranging in length from 222 to 332 mand in width from 
222 to 306 w. The spermatozoids of Ginkgo are described by Hirase 
(62, p. 123) as being egg-shaped and 82 4 long by 49 « wide. Those 
of Cycas are said by Ikeno (70, p. 580) to be 160 uw long by 70 » wide. 
The spermatozoids of Zama are thus much larger than those of Cycas 
