CLEAVAGE IN DIDYMIUM MELANOSPERMUM (PERS.) MACBR. I37 
adjacent to the shrinking protoplasmic surfaces (fig. 7). In sections 
this arrangement of the nuclei leads to their apparent distribution in 
rows along the radial clefts. This appearance in section, of course, 
is due to the fact that the nuclei really tend to occupy a layer of the 
cytoplasm next adjacent to the plasma membrane bounding the 
capillitial cavities. The majority of the nuclei appear in such rows, 
although a considerable percentage are irregularly distributed. In 
radial sections a row of the sort described is frequently to be seen in the 
protoplasm with no apparent capillitial opening near it. An exami- 
nation of the next adjacent sections, however, will always show that 
these rows are either immediately above or below a capillitial thread. 
The interesting fact is to be further noted that at the stage when 
the nuclei tend to show this characteristic arrangement along the 
cleavage surfaces a very high percentage of them are in the equatorial 
plate stage of division. This is especially true of the nuclei that have 
this characteristic position. Probably 75 per cent, of the nuclei at 
this stage that are arranged along the cleavage surfaces are in this stage 
of division. In figure 7 two of the nuclei in the equatorial plate stage 
are shown in polar view. The three resting nuclei may be distin- 
guished by the presence of their nucleoles. Deeper in the protoplasm 
the nuclei may or may not be dividing. The further fact may be noted 
that the long axis of the spindle is quite commonly more or less parallel 
to the surface of the protoplasmic mass adjacent to which it lies. 
This is true only of the nuclei lying near the plasma membrane ; those 
deeper in the protoplasm show no such orientation of their spindles. 
It is interesting to note that a very considerable portion of the 
nuclei in Didymium at this stage are in a resting condition and that 
these resting nuclei are scattered irregularly amongst those which are 
dividing. The conditions in this respect are in sharp contrast with 
those in Fuligo where, during the cleavage stages, in any particular 
region of the aethalial mass, all the nuclei will be found in division at 
one stage or another. It is also in contrast with the conditions in 
Enteridium and Lycogala in which practically all the nuclei in a 
particular spore sack divide simultaneously. Series of spore sacks 
adjacent to each other may be found in which every nucleus is in the 
equatorial plate stage of division. 
In Didymium it seems doubtful whether all the nuclei undergo 
division during the cleavage stages, though it is of course possible 
that the nuclei which are found in the resting condition at any par- 
