94 SPERMATOGENESIS AND FECUNDATION OF ZAMIA. 
it has reached a diameter as great as that of the pollen grain (fig. 13). 
The point where the tube bursts out of the pollen grain has also 
enlarged, the broken edges of the exine being bent outward. Mean- 
while the tube nucleus has assumed a round form, increasing some- 
what in size. 
The protoplasm presents a beautiful foam structure, with large 
vacuoles here and there. The starch grains which later fill the tube 
have not yet begun to appear. 
The next noticeable differentiation in the growth of the tube is the 
increase in size of the cells of the prothallus. Both cells increase in 
width and length and the first prothallial cell pushes out into the, 
second prothallial cell, which becomes shaped like a concayo-convex 
lens and is crescent-shaped in cross section. Figure 14 shows a pollen 
tube in the first stage of the development of the prothallus. The pro- 
thallus here has reached a size of 15 ~ wide by 16 w long. The nuclei 
of both prothallial cells have increased slightly in size and become 
spherical and less densely granular. The pollen tube is meanwhile 
gradually growing in length and diameter, the tube nucleus passing 
farther down as the tube elongates. In this tube (fig. 14) a dark line 
appears at the base of the prothallus which seems undoubtedly to be 
the remains of a third prothallial cell which has been resorbed. 
The pushing out of the first prothallial cell into the second prothallial 
cell is a point of considerable interest in clearing up the morphology 
of the prothallial apparatus, which was left in a very unsatisfactory 
state in the writer’s preliminary papers, as well as in the papers of 
Ikeno (70) and Hirase (62). In a somewhat later stage, when both of 
the prothallial cells have reached almost twice the size described in the 
last-mentioned stage, the first prothallial cell can be seen to have 
pushed a considerable distance into the second prothallial cell, the 
point of attachment of the plasma membrane of the cells still remain- 
ing in about the same relative position as in the original pollen grain 
(fig. 15). Meanwhile the second prothallial cell has arched out still 
farther, and by the increase in size of the first prothallial cell has been 
carried mainly out of the old walls of the pollen grain into the pollen 
tube. It may be remarked here that the prothallus still retains its 
original connection with the wall of the pollen grain, a connection 
which remains unbroken until the spermatozoids mature. In the 
preceding stage the nucleus of the second prothallial cell had increased 
in size slightly more rapidly than that of the first prothallial cell and 
had become slightly larger (fig. 14). In this stage (fig. 15) the second 
prothallial cell nucleus has become decidedly larger than that of the 
first prothalhal cell. 
DIVISION OF SECOND PROTHALLIAL CELL. 
Shortly after this stage the second prothallial cell divides into two 
very unequal cells, the stalk cell and central cell (kérper cell, generative 
