DEVELOPMENT OF THE MICROSPORES. 17 
In the formation of the two prothallial cells, Guignard shows that 
in each case it is the nucleus of the large cell which divides in cutting 
off the small cells of the prothallus. 
In the species of Zama studied by the writer, the mature pollen 
grain always shows two prothallial cells cut off at one side, and pro- 
truding into it (fig. t1). The development of the pollen has not been 
carefully studied, and the details of the formation of the prothallial 
cells is not known. It seems from the writer’s observation, however, 
that three cells are at least occasionally formed; and in this case the 
first one cut off is resorbed, as described by Strasburger (109) and 
others in Pinus, Ginkgo, etc., remaining as a dark more or less refrac- 
tive layer in the wall of the pollen grain situated at the point of con- 
tact of the other cells (figs. 11, 18, and 14). In many instances of 
mature pollen grains, and in later stages, during germination, no indi- 
cation of this resorbed prothallial cell can be observed, but in some 
cases it may be seen very plainly, and is unmistakable. 
In Ginkgo, according to Strasburger (109) and Hirase (62), the 
nucleus of the pollen grain undergoes normally three divisions, by 
which three prothallial cells are cut off, the first of which becomes 
compressed against the wall of the pollen tube and is largely resorbed, 
in the mature pollen grain appearing very indistinctly as a slight 
layer in the wall (Strasburger, 109, Pl. I, figs. 5 to 7, and Hirase, 62, 
figr,- 1). 
A careful investigation of the development of ‘the pollen of Zamia 
will have to be made before it can be determined whether three 
prothallial cells are regularly formed or whether the remnants of a 
third cell, occasionally observed, are to be considered as cases of rare 
and somewhat abnormal development. Judging from the normal 
occurrence of three cells in Ginkgo and Pinus, it would seem that 
probably three cells may also be normally formed in Zamia. How- 
ever, in the mature pollen grain, and in the pollen grains after 
germination in the nucellus, a third cell can only occasionally be 
observed, and the description here given will deal mainly with the 
two prothallial cells plainly evident in all cases. 
The nomenclature used here for the various cells of the antheridium 
is somewhat different from that usually used. It was thought best to 
use terms more in harmony with those used in the Pteridophytes in 
order to avoid confusion. The two prothallial cells normally cut off 
in the pollen grain are distinguished in the order of their formation 
as the first and second prothallial cells (P1 and P2). When the 
second prothallial cell divides it gives rise to the stalk cell and central 
cell (Kérper cell, body cell, generative cell, etc.). The cell here called 
the central cell is considered by Strasburger (109, p. 7) and others as 
corresponding to the central cell of the antheridium in ferns. The 
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