274 Reed . — -The Nature of the Double Spireme in Allhtm Cepa . 
in size of the nuclear vacuole, so much so that Lawson (10, 11, and 12) has 
termed these prophases the ‘ growth period \ 
During the early stages of the prophase the nuclear vacuole has a very 
definite boundary — the so-called nuclear membrane. The growth of the 
vacuole is accompanied by the stretching or by the growth of this boundary 
to keep pace with the increased volume. Whilst the distension of the 
vacuole is progressing, the chromatic elements become closely pressed to its 
surface, so that there is little doubt that this growth is a result of increased 
turgidity due to the metabolic processes which must be taking place, and 
which result in the formation of the spireme segments. 
The increased size of the nuclear vacuole is accompanied by a corre- 
sponding diminution in definiteness of its boundary. As the older stages of 
prophase are reached, the nuclear vacuole occupies almost the whole of the 
cell space, and some of the chromosomes have passed out into the general 
cytoplasm (PI. XVIII, Figs. 9, 10, 11). Finally, the limits of the vacuole 
appear merely as a faint, irregular boundary of cytoplasm (PL XVIII, 
Figs. 9, 10, 14, 15, 16) without the slightest suggestion of a definite mem- 
brane as a distinct entity, which is suggested by Lawson (11), Nemec (18), 
Merriman (15), and others. 
Further, the form of the vacuole is not necessarily ‘ perfectly spherical \ 
as described by Lawson (p. 154, 10) for A. Cepa. It may be quite irregular in 
outline, and is sometimes even bilobed (PI. XVIII, Fig. 1 b), although this 
latter condition is not of frequent occurrence. Farmer (2) has shown that 
the form of the vacuole will not be determined by osmotic pressure alone, 
but that surface tension will play a part in the determination of its shape. 
When the vacuole has reached its full size Lawson maintains that it 
collapses and that the nuclear membrane becomes wrapped around each 
chromosome. The collapse, according to this author, is followed by the 
drawing in of the cytoplasm, which is consequently compelled to assume 
a radiate structure, in which he sees the beginnings of the formation of the 
achromatic spindle. 
No evidence whatever is found in the present investigation which 
supports these conclusions. The ragged edge of the cytoplasm can be 
recognized even when the chromosomes are passing on to the equatorial 
plate (PI. XVIII, Fig. 16). 
This is not the place to discuss the bearing of the above conclusion on 
the synaptic contraction during the heterotype mitoses, but it may be 
remarked that no useful purpose would be served by giving series of measure- 
ments showing the increase in size of the nuclear vacuole or the volume of 
the chromatin mass. This method of comparison would only be useful if the 
nuclei were of similar sizes and if they contained equal volumes of chromatin. 
In the somatic nuclei of A. Cepa it can hardly be assumed that the latter 
supposition is valid, whilst the former can easily be shown to be untrue. 
