996 M. M. Mercatr 
The nuclear membrane never disappears even during mitosis. 
The nuclear membrane shows no structure. Under all conditions, 
whether living, or after treatment with acetic acid, silver nitrate, 
or fixing agents without staining, after all sorts of staining in total 
preparations or in sections, one finds it always appearing homogeneous 
and uninterrupted. There are no indications that the achromatic 
structures in the nucleus are in any way continuous through the 
nuclear membrane with the structures of the cytoplasm. Of course 
in each division the membrane is ultimately broken at the point of 
constriction, but this break occurs in the slender connecting thread 
at a distance from the cavities of the daughter nuclei and there are 
no wounds at the surface of either nucleus. 
The nucleus lies in the cytoplasm, as it were in a great alveolus, 
being suspended and held in place by the films of the cytoplasmic 
foam. Fig. 205, Pl. XXIV, gives a clear picture of this condition in 
the case of a male pronucleus in a zygote of O. intestinalis. 
The resting nucleus (Pl. XX, Figs. 76 and 77). 
The phrase “resting nucleus” of course does not imply that the 
nucleus is inactive physiologically, but only that it is not engaged 
in the movements which constitute or accompany mitosis. One can 
hardly speak with propriety of such a resting stage in the nuclei 
of O. intestinalis, for there seems to be no time throughout the year 
when changes in their visible structure are not constantly occurring. 
The divisions of the nuclei and of the body never cease, and every 
nucleus seen is either in actual division or is preparing for or 
recovering from division. There is no condition in which the nuclei 
seem to pause for any prolonged period. The stage which corresponds 
to the ordinary resting nucleus of metazoan cells is probably that 
in which the chromatic network is most branched and diffuse. I will 
begin the description with the stage just prececding the formation 
of the mitotic spindle. 
Achromatic foam. 
The whole space within the nuclear membrane is seen to be 
filled with alveolar protoplasm, the alveoles in many places being 
fused to form vacuoles of different sizes (Fig. 77. Pl. XX). In many 
other nuclei the alveoles are not fused but are of fairly uniform 
size (Fig. 69, Pl. XIX). Granules of varying sizes and irregular 
