Opalina. 299 
One of course must inquire as to the meaning of this puzzling 
difference between the two species. I can suggest no adequate ex- 
planation. The nuclei lie much further forward in O. intestinalis 
than in O. caudata (compare Fig. 38, Pl. XVII, with Fig. 81, Pl. XX). 
It suggests itself that the nucleolus in both species may remain in 
the nucleus which is nearer to the center of the body, or rather 
nearer to the protoplasmic rather than the geometric center of the 
body. The geometric and protoplasmic centres are not the same, 
for the protoplasm in the anterior part of the body is more dense 
than that further back. This suggestion fits the conditions in 
O. intestinalis and, for the most part, also, the conditions in O. caw- 
data. If one takes into account the greater density of the anterior 
part of the body, it is true that in O. caudata in the cases in which 
the old nucleolus remains in the anterior daughter nucleus it is 
nearer the center of the protoplasm. I have looked through many 
preparations, comprising many hundreds of O. caudata, to see if in 
cases in which the old nucleolus remained in the posterior daughter 
nucleus the nuclei were unusually far forward, so that the proto- 
plasmic center might in these cases be nearer to the posterior than 
to the anterior nucleus. In the majority of instances of this sort 
it was found that the nuclei were unusually far forward, almost as 
much so as in O. intestinalis, but, unfortunately for my suggestion, 
no less than six instances were found in which the old nucleolus 
remained in the posterior daughter nucleus, although this was placed 
not only not exceptionally far forward but even unusually far back. 
I believe, therefore, that there is probably no worth in the suggestion 
made. 
The new nucleolus in the daughter nucleus of O. intestinalis 
arises always at the pointed end of the nucleus near the thread 
which connects it with its sister nucleus (Fig. 72, Pl. XIX). At 
first it is very small. It grows rather slowly, but by the time the 
nucleus is ready for its next division the nucleolus is again of 
full size. 
I have little suggestion to make as to the nature or function 
of the nucleolus. I would merely emphasize 1) that it is wholly 
distinct from the chromatin elements and never at any time has 
any discernable genetic relation to them; 2) that a nucleolus once 
formed persists throughout the year and until the nucleus con- 
taining it is ready to throw off its vegetative chromidia and enter 
upon sexual reproduction (processes which will be described later). 
I have not found nucleoli in the nuclei of any forms between the 
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