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 close 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. 



Chromatiti. — 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 catenulatum. 



