24 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



and extends to the peripheral ones on the membrane (figs. 

 48 and 50, «^). This centralization of the nuclear contents 

 would seem to be a shrinkage product of fixation, if speci- 

 mens like those shown in fig. 35 had not been repeatedly 

 seen lying side by side in material fixed on the slide with 

 osmic vapor, picro-carmine, etc. There seems to be a 

 stage immediately preceding and following division, when 

 the chromatin is more evenly distributed through the nu- 

 cleus, the whole presenting a more homogeneous appear- 

 ance, while the intervening periods are characterized by the 

 arrangement described above. 



Nucleoli. — In many sections, a non-staining material is 

 seen in rounded or lenticular masses at the ends of the 

 chromatin aggregation, more rarely at the sides (figs. 53- 

 55, e). This substance has a pale yellowish gray color in 

 sections from material fixed in picro-acetic, Hermann's or 

 Flemming's fluids. These masses have rarely been seen 

 in sections of division stages, and then not occupying any 

 significant position, such as is assumed by the " Pol- 

 platten " in Sfirochona, Paramcecimn, etc. Possibly they 

 are comparable to Hertwig's plastin nucleoli in Actino- 

 sphcBrium. 



Micronucleus. — Near the aboral end of the organism is a 

 small, very definite and constant micronucleus (diameter 

 1.5 ytt), staining readily with iron-hgematoxylin, methyl- 

 green, picro-carmine, etc. (fig. 48, w). Around it in 

 sections is a narrow clear space, bounded by the sem- 

 blance of a membrane, scarcely more than the alveolar 

 walls in resting stages, but more definite in early division 

 stages, and disappearing in the final stages of division. 

 This more or less definite limiting boundary of the peri- 

 micronuclear space seems to be not a homogeneous secre- 

 tion, but to be due to greater or less compression of the 

 surrounding alveoli. It is, however, very constant. 



Division Phenomena. — The phenomena of nuclear divi- 

 sion were first observed in fresh material treated on the slide 

 with picro-carmine, — this reagent simultaneousl}' fixing and 



