920 Gates.—Pollen Formation in Oenothera gig as. 
PL LXVII, Figs. 15-19 represent the same stage of the nucleus, in 
which a very characteristic structure is exhibited. This condition also follows 
soon after synizesis, and a complete rearrangement of the material from 
the synizetic condition has taken place. Figs. 15-17 are all sections of the 
nucleus at this time, representing its structure in different ways. The threads 
passing through the cavity of the nucleus are at this time comparatively few, 
as shown in Fig. 15, while the greater part of the chromatic material forms 
a rather delicate and very characteristic threadwork lining the nuclear mem¬ 
brane. Many of these threads appear in cross-section in Fig. 15. They 
adhere so closely to the nuclear membrane as to appear to be almost 
embedded in its substance. These threads are c chromomeric ’ in structure 
throughout. They are nearly as delicate as in synizesis, and they are 
certainly single, not double, and contain one row of ‘ chromomeres ’. This 
threadwork lining the nucleus frequently gives the appearance of a double 
nuclear membrane, i. e. one membrane within the other, but by careful 
focusing it can always be shown that the two apparent membranes in one 
focus merge into one. Figs. 18 and 19 are surface views of a portion of the 
nuclear membrane, to show the nature of this lining threadwork. Fig. 18 
shows that free ends of the threads sometimes occur. This threadwork lines 
the whole of the nucleus in a remarkably uniform manner. The nuclear 
membrane itself is very definite at this time. As already stated, relatively 
few threads pass through the centre of the nucleus, and these are mostly 
different in structure from the threadwork lining the nuclear membrane. 
These central threads vary greatly in thickness, as shown by Figs. 15-17, 
some of them being coarse and heavy masses, while others are thin and 
delicate. In Fig. 17 a different representation of the chromomeric threads 
was attempted. 
During this period the nucleolus varies in shape, frequently showing 
a certain amount of distortion, or being characteristically flattened as in 
Fig. 19. 
Whether Figs. 15-19 represent a stage of development through which 
the nucleus always passes, or whether they represent rather a physiological 
condition of the nucleus which may or may not occur, is uncertain. In any 
case, it is a very characteristic condition, and exhibits a distribution of the 
chromatic material of the nucleus which is quite the reverse of that seen in 
synizesis, for instead of being rather closely aggregated in the synaptic ball 
it is in considerable part arranged as a threadwork lining the nuclear mem¬ 
brane. Davis (’ 09 ) has figured what is evidently the same stage in 
O . grandiflora (PI. XLI, Fig. 13), but does not explain it. 
The figures presented show how numerous are the stages or conditions 
of the nucleus during the period of synapsis. As I have shown elsewhere 
(Gates, Tl), there is no reason to believe that synapsis accomplishes any 
special exchange of materials or influences between homologous chromo- 
