No, 2 .] COMPARATIVE CYTOLOGICAL STUDIES. 
445 
There is without doubt in this genus, as in the other meta- 
nemerteans, an extranuclear origin of the nucleolar substance. 
This is proved (i) by the absence of nucleoli in the nuclei 
from which the germinal vesicles are derived ; (2) by the 
nucleoli first appearing dose to the nuclear membrane. And 
since yolk globules do not arise in the cytoplasm until nearly 
the close of the second nucleolar period,when most of the nucleoli 
are near the center of the nucleus, to the yolk substance cannot 
be attributed a nucleolar derivation, and other reasons, such as 
the fact that the yolk balls usually appear at some distance 
from the nucleus, would contradict such an assumption. The 
nucleolar substance is apparently formed from an unstaining 
fluid constituent of the cytoplasm, which after it is taken into 
the nucleus undergoes a chemical change, since it stains there 
and is deposited in the form of nucleoli. In the second nucleolar 
stage, when the formation of nucleoli is at its height, the 
nuclear sap stains more deeply than at any other period (Figs. 
224-227, 233), so that it is probable that at this time nucleolar 
substance is finely distributed throughout the nuclear sap, as 
well as in the form of nucleoli. (This staining of the nuclear 
sap is especially well seen on material fixed with Flemming’s 
fluid and stained with alum carmine.) 
In the third and fourth nucleolar stages a few yolk globules 
are often found in a number of germinal vesicles (Figs. 234 and 
235, Yk. Gl .); these have probably been taken up by the nucleus 
from the cytoplasm. 
Chromatin. — In the nuclei of the first stage, the chromatin 
is always demonstrable in the form of coarse granules (Figs. 
214, 216, 219). In the beginning of the second it may usually 
be found in the form of a reticulation (Figs 218, 229, 233), but 
at the end of this stage it is not demonstrable (Fig. 227). In 
the third and fourth stages it reappears, but now in the form 
of fine microsomes (Fig. 235); and at the conclusion of the 
fourth stage short chromatic filaments begin to arise, similar 
to those described for Tetrastemma catcnulatum. 
