No. 2.] COMPARATIVE CYTOLOGICAL STUDIES . 
431 
the nuclear membrane has attained its greatest thickness. The 
thinness of this membrane in previous stages would allow the 
penetration of nutritive substances into the nucleus from 
the cytoplasm. The small nuclei from which the germinal vesi¬ 
cles are directly derived, without any intervening mitoses, are 
irregular in shape, and no nucleoli are to be seen in them 
(Figs. 108 and 112, C. T. N.). 
5. Tetrastem?na elegans (Verr.). 
(Plate 28, Figs. 282-299.) 
Having only two mature individuals of this worm for study, 
I am unable to give as thorough a description of the nuclear 
metamorphoses of the egg as was possible for the other nemer- 
teans ; one preparation was fixed with Hermann’s fluid, the 
other with aqueous solution of corrosive sublimate, but the 
latter had been too deeply stained (haematoxylin, eosin) to 
allow the study of certain details, as eg., the cytoplasmic 
changes leading to the formation of the yolk. Yolk balls were 
observed in only a few ova, and are much less numerous than 
in T. catenulatum ; it is possible that the development of the 
yolk in the present species may be as in Zygonemertes , that is, 
the mature yolk spherules may as a rule be directly formed 
without the interpolation of a yolk-ball stage. 
First nucleolar stage. — The youngest germinal vesicle, recog¬ 
nizable as such, showed a large nucleolus close to the nuclear 
membrane (Fig. 282) ; I have seen no smaller nuclei than this 
one, but would conclude by analogy from the facts in the other 
metanemerteans that also here all the nucleoli have an extra- 
nuclear origin. In slightly larger nuclei (Figs. 283-287) there 
are from one to three nucleoli, whose size varies considerably 
with regard to that of the nucleus, as well as to the size of one 
another. In such cases (Fig. 283) where only two nucleoli are 
present, one near the center of the nucleus, the other close to 
the nuclear membrane, the former is probably the older and 
has left the periphery for the center of the nucleus, while the 
other is younger and is still in process of formation. These 
first-formed nucleoli are usually rather large in proportion to 
