Gates.—Pollen Formation in Oenothera gigas. 923 
in the median plane of the heterotypic spindle. It seems to disappear with 
the spindle fibres, however. 
Homotypic Mitosis. 
Fig 28 shows the beginning of homotypic spindle formation, the first 
indication being the development of a weft of delicate wavy fibrils surround¬ 
ing the nucleus. These become drawn out at certain points, as in Fig. 29, 
and then the nuclear membrane disappears. As will be seen from Fig. 28, 
the chromosomes in the homotypic prophase, before the disappearance 
of the nuclear membrane, show clearly their bivalent structure, which 
differs in no respect from that of the heterotypic telophase. 
Just as in the anaphase and early telophase of the heterotypic mitosis 
the chromosomes not infrequently show no indication of a bivalent structure, 
so in the prophase of the homotypic mitosis this is sometimes the case. 
In Fig. 30 only one of the chromosomes is clearly bivalent, the rest showing 
little or no indication of a bivalent structure. Fig. 31 shows both the 
homotypic spindles in late prophase, just before the chromosomes are 
drawn into the equatorial plate. Nearly all of these chromosomes show 
their bivalent character. In this section all the chromosomes of the cell are 
present, there being sixteen on one spindle and twelve on the other. This 
means that on the heterotypic spindle in this mother-cell a 12-16 distri¬ 
bution of the chromosomes occurred, which is the widest departure from 
an equal distribution that I have found. Several cases of a 13-15 distri¬ 
bution in the reduction division were observed by counting the chromo¬ 
somes in the early heterotypic telophase. 
Fig. 32 is a side view of the homotypic metaphase, showing the 
chromosomes so closely massed that their boundaries are not clearly 
distinguishable. Fig. 33 shows all the chromosomes of a mother-cell 
in two rather scattered groups of fourteen each, probably in polar view, 
although the spindle fibres were not visible, and. in this case there was 
no indication of the bivalent character of the chromosomes. Fig. 34 shows 
the homotypic spindles in side view, the chromosomes all showing their 
bivalent structure very clearly in this case. They are just being oriented in 
the equatorial plate, and several of them are still out of alinement. In 
Fig. 35 the chromosomes cannot all be separated from each other, and they 
are rather irregularly arranged on the spindles. In Fig. 36 only two of the 
chromosomes—one on each spindle—show their bivalent character. Their 
alinement is still somewhat irregular. PL LXIX, Fig. 37 is an early anaphase, 
in which the halves of the chromosomes have in most cases just separated. 
In this figure several of these anaphase chromosomes show a median 
constriction. I shall refer to this again later. In Fig. 38 the spindle to the 
left is in side view, showing several of the chromosomes whose halves have 
not yet separated, while the spindle to the right is seen in slightly oblique 
