560 D avis —Cytological Studies on Oenothera . /. 
segmentation of the spirem the loops (now rings) are joined together like 
links in a chain. 
(6) It seems impossible in this form to divide the phases of synapsis 
into periods of a first and second contraction as has been done in some 
other studies. The changes of structure during synapsis are so gradual as to 
make sharp divisions between the periods very difficult to define. Further- 
more, the degree of variation in the form of the chromatic material at the 
same phase of synapsis makes it unsafe to assume that looser types of 
synaptic contraction indicate periods of expansion to be followed later by 
further contractions. 
The Heterotypic Mitosis. 
The prophases of spindle formation of the first, or heterotypic, mitosis 
in the pollen mother-cell appear shortly after the seven chromatic rings, or 
bivalent chromosomes, emerge from the synaptic knot and are distributed 
through the nuclear cavity presenting the stage called diakinesis (Figs. 28 
and 31). The rings by this time have grown smaller, that is, their substance 
has become more condensed, as may be seen by comparing Figs. 31, 32, and 
33 with Figs. 25, 26, and 29. The halves of the rings soon become more 
sharply defined and gradually take on the bent or V-shaped forms, 
characteristic of the sporophytic chromosomes with which they are identical. 
These half-rings, or sporophytic chromosomes, however, remain connected 
in pairs so that the rings are still intact. It is clear that the process of 
condensation and differentiation continues through the stages of prophase 
up to the period of metaphase when the two sets of sporophytic chromo- 
somes are ready to be distributed by this, the heterotypic mitosis. 
The process of spindle formation in its main features follows a history 
similar to that described for the heterotypic mitosis in a number of higher 
plants (e. g. Equisetum,Larix,Lilium , &c.). The nucleus becomes surrounded 
by a web of fibrillae which enter the cavity of the nucleus with the breaking 
down of its membrane (Figs. 31, 32, and 33). The large nucleolus at this 
time stains faintly and soon disappears. The developing fibrillae gradually 
fill the nuclear cavity, and at the same time push their way out into the cyto- 
plasm in several directions, giving the well-known multipolar stages of spindle 
formation. Sections of prophases showing three or four poles are common 
(Figs. 35 and 36). 
The chromatic rings are carried towards the centre of the developing 
mass of fibrillae, where at first they frequently lie in a group so closely 
massed that their arrangement can be made out only with difficulty (Fig. 35). 
However, one or more of the rings are generally apart from the main group 
and their forms may be clearly seen (Figs. 34 and 36). 
The spindle at metaphase of mitosis (Plate XLII, Figs. 37 and 38) is 
a rather broad bipolar structure developed from the multipolar conditions 
