Division of the Generative Nucleus in Pines. 209 
contracted at or near its centre during its disintegration, thus 
presenting the appearance of an hour-glass, or it may give 
rise to such a condition as that shown in Fig. 42 . This 
figure with various modifications is not uncommon. I am 
unable to trace its origin definitely, but it is not improbable 
that it is caused by a contraction of the cytoplasm resulting 
from a cessation of the force which tends to separate the 
daughter-nuclei ; or it may be caused by the resistance which 
the peripheral layer of cytoplasm, along the outer surface of 
the upper nucleus, offers to the growing fibres, thereby forcing 
them back upon themselves as shown in the figure. When 
all traces of the spindle have disappeared, the two sperm - 
nuclei are surrounded by a common mass of cytoplasm. 
The mitosis just described seems to be unique as regards 
the origin and development of the achromatic spindle. The 
most exaggerated instance of asymmetry in spindle-formation 
which I have found recorded is that described and figured by 
Nemec (’99, 2) in Solanum tuberosum. Here the nucleus lies 
at one side of the cell, and the spindle-fibres are very much 
more prominent on the free side of the nucleus than on the 
side adjacent to the cell-wall. In another paper Nemec 
(’99, 3) shows by experimentation that the form of the figure 
which gives rise to the extranuclear spindle depends upon 
external forces or conditions. In the case of the pines 
studied, the position of the generative nucleus is such that 
the spindle if extranuclear in origin must of necessity be 
unipolar, since there is no cytoplasm, or almost none, above 
the nucleus from which fibres could arise. 
The blending of the linin reticulum with the cytoplasmic 
network after the disappearance of the lower portion of the 
nuclear membrane, and the relation of certain portions of the 
achromatic nuclear reticulum to the ingrowing fibres, suggest 
an intimate relation between these structures. That the 
spindle-fibres which originate in the cytoplasm and apparently 
grow by a differentiation of its network are later fed by the 
linin of the achromatic nuclear reticulum there seems little 
room for doubt. In fact, all the phenomena connected with 
p 
