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 1 12, C. T. N.). 



5. Tetrastemma 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 e.g., 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. caieniilatii7n ; it is possible that the development of the 

 yolk in the present species may be as in Zygoncmertcs, that is, 

 the mature yolk spherules may as a rule be directly formed 

 without the interpolation of a yolk-ball stage. 



First nucleolaj' 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 



