26 SPERMATOGENESIS AND FECUNDATION OF ZAMIA. 
(fig. 18). This stage 1s a very easy one to find and the writer has 
many sections showing it very plainly. In the tube shown in figure 18, 
which is only shortly after the completion of the division, the nuclei 
of both the stalk and central cells have assumed a rounded form, the 
latter being much the larger. The nucleus of the central cell is here 
9.79 4 in diameter while that of the stalk cell is only 7.12 m and 
that of the first prothallial cell about 8.9 jin diameter. The entire 
prothallus in this stage immediately after the division is only 29.37 ju 
long by 16.91 4 wide. The first prothallial cell is now almost entirely 
surrounded by the stalk cell, only the base of the cell remaining in its 
original position. A few small starch grains have already begun to 
appear in this cell, two being shown in the figure. In later stages 
both this cell and the stalk cell become crowded with starch. Before 
proceeding farther it will be desirable to point out the views held in 
regard to the structure of the prothallus in Cycadacee and Ginkgo in 
previous publications. Attention was first called to the peculiar 
structure of the prothallus in the Cycadacee in the writer’s first pre- 
liminary paper on Zamia (122). Here it was stated: 
The former cell [in reality the staik cell, as proved by later researches] is spherical 
or slightly elongated and presents a most singular structure. The nucleus of the 
original cell evidently divides into two, and one of the daughter nuclei forms within 
the unbroken Hautschicht of the mother cell a new and wholly distinct Hautschicht, 
which delimits a cell lying entirely free within the mother cell and surrounded 
on all sides by a layer of protoplasm of nearly uniform thickness (figs. la and 2). 
The other daughter nucleus remains free within the Hautschicht of the mother cell, 
but is pressed to one side by the interior cell. ; 
Tt will be seen from this that the writer was greatly in error in his 
early interpretation. This was largely due to the fact that sections 
must be exactly median longitudinal through the pollen grain and _ 
prothallial apparatus to show that the first prothallial cell (interior 
cell) has any connection with the wall of the pollen tube. Cross 
sections of the tube which were then used considerably in the writer’s 
investigations also fail to show the true relationship of these cells. 
Their confusing structure in a section of this kind will be seen by 
examining figure 23. The difficulty of the investigation leading to the 
correct interpretation of this structure is also shown by the views 
expressed by the Japanese authors which are at great variance with 
those of the writer. | 
In Ginkgo the first prothallial cell, which the writer has found to 
become surrounded by the stalk cell through transformation during 
growth, Hirase considers to be simply strands of protoplasm in the 
second prothallial cell. He says: 
At the extremity of the tube, which is covered by the exine and extends into 
the cavity, are found in the interior two flattened prothallial cells which are now 
separated from each other. Between these there are large vacuoles, and it may be 
seen also that they are united by the cytoplasm which forms two hollow cylinders 
placed one within the other, so that if a section be made along the longitudinal 
