562 Davis. — Cy to logical Studies on Oenothera. I. 
a heterotypic spindle a vacuole-like region, the boundary of which becomes 
the membrane of the daughter-nucleus. The seven split chromosomes, 
which were at first massed closely together, separate as the daughter- 
nucleus gradually increases in size, and become distributed rather symmetri- 
cally around its interior just under the nuclear membrane. A change in the 
form of the split chromosomes becomes then at once apparent. The ends 
of one chromosome of the pair swing away from the ends of the other until 
they lie in approximately the same plane, when the structure becomes that 
of two U’s joined together at the bent middle regions. This condition 
is illustrated in Figs. 44 and 45, which also show the development of 
the nucleoli that are conspicuous in the full-sized resting nucleus. 
A careful study of stages makes it perfectly clear that the halves 
of the split chromosomes are not crossed, even though the form of the 
double structure is sometimes like that of an X, but that they lie side 
by side joined at the bent middle regions. This striking association of 
chromosomes is very characteristic of these resting nuclei. There is 
sometimes a tendency on the part of the free ends of the chromosomes 
to extend and to branch, and even to become united with one another, 
thus forming a loose and imperfect network (Fig. 43). However, I have 
never seen a resting nucleus in which the outlines of one or more 
chromosomes could not be accurately followed. Fig. 45 shows a pair of 
nuclei that are fully at rest ; in the lower one of the two the seven 
split chromosomes are entirely distinct from one another and may be 
counted, and in the upper one (cut by the knife) four of the split 
chromosomes may be readily distinguished. 
There can be no doubt that in this type the chromosomes maintain 
their individuality in the resting nucleus between the heterotypic and 
homotypic mitoses. This conclusion is further supported by the structure 
of the chromosomes as they appear in the early stages of the homotypic 
mitosis to be described below. The point is one of considerable impor- 
tance because of the views held by some plant cytologists that the 
chromosomes may entirely lose their individuality in the resting nucleus. 
In conclusion, it is evident that the heterotypic mitosis distributes 
the fourteen sporophytic chromosomes in two groups of seven each, and 
consequently is a reduction-division. These sporophytic chromosomes are 
grouped in pairs as ring-shaped bivalent chromosomes which appear 
after synapsis, and are characteristic of diakinesis and the prophases of 
spindle formation. The sporophytic chromosomes are V-shaped at the 
metaphase of mitosis and appear essentially similar to one another. There 
seems to be as great uniformity in their shape and size as is common for the 
heterotypic mitoses of plants. 
