380 Gates and Rees . — A Cy to logical Study of 
cell’s own internal activity. It is well known that each genus of higher 
plants often has its own characteristic form of pollen grain, and it is 
perfectly certain that the steps towards the assumption of that form which 
are taken while still within the mother-cell wall must be determined 
internally by heredity. Whether the later sculpturing of the pollen grain 
wall is also internally determined, or externally, by apposition of material 
from the remains of the tapetum, is uncertain. In how far the latter process 
may take place in the later stages of pollen development has never, we 
believe, been satisfactorily investigated. But it is clear that the early 
changes of shape which take place in the young grains within the mother- 
cell are internally determined in all cases. This was shown, for example, 
in Oenothera (see Gates, 1911 , PI. LXIX, Fig. 47, 48), in which the young 
pollen grains separate from the centre of the mother-cell, and form within 
its periphery four discoid cells circular in surface view before the mother- 
cell wall breaks down. Later the characteristic lobes grow out from three 
equidistant points in the circular edge of the flattened discoid cells. These 
are also no doubt internally determined, as is proven, for example, by the 
fact that while the pollen grains of diploid species have three lobes, those of 
tetraploid species have four or sometimes more. 
By the time the young pollen grains in Lactuca have assumed their 
roughly heptagonal shape, a thin cell membrane has already been secreted 
around each by the plasmatic membrane of the cytoplasm (Fig. 68). This 
wall rapidly thickens and develops symmetrical outgrowths (Figs. 70-71) 
until it reaches the mature stage shown by the grains in Fig. 72. There 
is a median frill, pinked at the edges, with usually seven, sometimes five, 
denser radial bands, which are optical sections of other ‘ frills ’ running 
meridionally. In addition, areas of the cell-wall become greatly thickened 
to form an approximately circular outline. Whether this complicated and 
nearly symmetrical system of thickenings is laid down by apposition from 
the tapetal plasmodium we have not determined, but the irregular margin 
of the outer thickening suggests that this may be the case. Nevertheless, 
the general arrangement of the thickenings must clearly be controlled from 
within the cell, and probably the same is true of the circular thickening. 
Irregularities. % 
Under the heading of irregularities are included (1) an account of 
pollen mother-cells which are binucleate before, during, or after synapsis, 
and (2) the occurrence of cytomyxis. 
A. Binucleate Pollen Mother-cells. 
An interesting feature of the pollen development is the occasional 
occurrence of binucleate mother-cells. Several such cells are represented 
in Figs. 14-17. In each of them the two nuclei are in exactly the same 
