Pollen Mother- cells of Li Hum canadense . 243 
appearance in the young nucleus of two nucleoles, which, if a long resting 
period ensues, commonly fuse into one. In the primary spermatocytes 
of Diaptomus two nucleoles appear in the early prophases and become 
fused during synapsis. This seems to be the only case he has discovered 
in which a fusion of nucleoles occurs during the prophases ; and this 
unusual occurrence during this particular mitosis in Diaptomus may very 
well be a manifestation of the same general tendency toward a fusion of 
two groups of nuclear substances which shows itself, apparently universally, 
in a rearrangement of the chromatin. The fusion of the two spirems 
involves a change in the position of the material of one or both ; it would 
be extremely interesting to determine whether there is a mutual ap- 
proximation, or whether in this respect one is passive, the other active; 
but upon this point I have no evidence to offer. 
The aggregation of the nuclear material begins, as I have said, in the 
very early stages of the formation of a uniform spirem ; the movement, 
then, is not by the fully-formed spirem threads, but, in part at least, by the 
knots and fibrous material which constitute the reticulum. This fact also 
appears from the not infrequent occurrence, in the formation of the spirem, 
of two short threads ending in the same knot ; such figures may well result 
from a previous fusion of the knots in pairs. But this occurrence is by no 
means general. The fusion of the two threads is followed by the fusion of 
the chromomeres in pairs ; this fusion is not completed for some time after 
the threads have come in contact, but finally no evidence remains in the 
appearance of the chromomeres of their double origin. Occasionally 
a chromomere seems not to find a mate, but such occurrences are excep- 
tional ; and the most striking fact apparent in the approximation of the 
threads is that they come together in such a way that in general each 
chromomere in one thread lies opposite a chromomere in the other thread. 
All appearances indicate that the object of the whole process is the fusion 
of the chromomeres. The result, therefore, seems to be the bringing into 
at least intimate contact of the chromatin derived respectively from the two 
parental germ nuclei. In speaking of the fusion of the chromomeres I do 
not mean to imply that I have observed any indication of the actual fusion 
of the smaller granules of which the chromomeres are composed ; all that 
is apparent is a flowing together of two masses of granules into a single 
mass. How the number of granules present after the union compares with 
the total number before it is impossible to determine ; plainly there are 
several conceivable possibilities in the case, which will be discussed in 
another connexion. It is sufficient for the present to say that the fusion of 
the two spirems effects that intimate association of diverse hereditary 
substances which was anticipated in the fusion of the parental germ-cells 
some hundreds or thousands of cell generations previously. 
After the completion of the union of the two threads, the now seem- 
