390 
CLIFFORD H. FARR 
Upon it probably becomes rounded at the end, of a more nearly uni- 
form width, and finally may recede somewhat. In case it recedes, it 
doubtless becomes broadened to a greater extent (fig. 9). In some 
mother cells in the homoeotypic karyokinesis there appears to be very 
little if any furrow at all, whereas very few cells are to be found in 
which two fully developed nuclei occur without some sort of a furrow 
being present. All of which is evidence that a furrow is always formed 
during interkinesis and then may recede even to obliteration. 
The cell-wall material always fills the furrow and appears as a jelly- 
like mass. The second division furrow is always narrow in section 
(figs. 13 to 18), and never broadens at the base like that of the first 
division. It is thus reasonable to assume that when the furrows begin 
to develop after the homoeotypic nuclear division, they progress con- 
tinuously until the cell is divided, in this way differing from the first 
furrow which is known to be arrested in its development for a time. 
This sharp-edged furrow is the only kind that is to be found after the 
second division, and it is entirely probable that the heterotypic furrow 
begins in this way, as suggested above. If we think of the plasma 
membrane as the active agent in furrowing, and the attraction be- 
tween it and the nuclear membranes as the force involved, this tension 
would be destroyed upon the disintegration of the nuclear membranes 
in the prophases of the homoeotypic karyokinesis, and the cell turgor 
would express itself in pressure against the plasma membrane, causing 
the equatorial furrow to recede or at least to flatten out somewhat. 
Conklin (5) and others have shown that in certain animal cells the 
cleavage furrows are arrested as a result of the cells being placed in 
hypertonic solutions, while the division of the nuclei and centrosomes 
may continue. 
After the homoeotypic nuclear division, the daughter nuclei are 
at first narrow in section and close together (fig. 23), just as is the case 
after the first division. As the spireme passes over into the prochro- 
mosome stage the sister nuclei enlarge (fig. 12), become more nearly 
spherical (fig. 11), and pull apart slightly, but not so much as in the 
heterotypic division (fig. 15). No orange zone (fig. 23) or other sem- 
blance of an equatorial differentiation in the central spindle is found 
in the homoeotypic division. As soon as the nuclear membranes are 
formed, fibers appear across the heterotypic equator and soon definite 
spindles are organized. Because of the proximity and narrowness of 
the daughter nuclei, at first these spindles are not completely organized ; 
