208 Sargant. — Formation of the Sexual Nuclei 
some time the envelope of the pollen-mother-cell is still 
visible round each group of four. 
We have now traced the history of the four pollen-grain 
nuclei through all the steps of their development from the 
nucleus of the pollen-mother-cell. We have seen that the 
chromosomes of this nucleus showed at one period a com- 
plete division into two segments and an incomplete division 
of each segment into two. We have seen the two segments 
separated from each other during the first karyokinesis of the 
pollen-mother-cell nucleus. It is natural to inquire whether 
the subsequent division of these segments in the second 
karyokinesis does not correspond with the incomplete fission 
visible in each segment before the first took place. This 
question, however, I am unable to answer. It is difficult to 
believe in a sudden and complete change of structure in each 
chromosome at the time when its colouring becomes uniform 
(p. 202). But no trace of fission can be seen in the segments 
after that period. It might be possible to trace it from one 
karyokinesis to the other by microchemical methods, for there 
is no intervening period of rest, but the few experiments I have 
tried gave no conclusive result. Whether the daughter- 
segments separated during the second karyokinesis are 
formed de novo shortly before separation, or whether they 
date back to an earlier period, they are equally the result of 
longitudinal fission. 
Formation of the Male Pronucleus. 
The young pollen-grains are soon set completely free within 
the loculus of the anther by the disappearance of the mother- 
cell envelope. No further change takes place in the nucleus 
of each grain for some time, during which the exine becomes 
thickened and beautifully sculptured on the outer surface 
(Figs. 23 and 27). The pollen-grain meanwhile grows con- 
siderably in length and assumes a curious shape (Fig. 23 a 
and b). The resting-nucleus can hardly find room within it, 
and is often flattened (Fig. 23 b). There is a deep fold in the 
