306 . BULLETIN OF THE 



In his work on the development of Elasmobranch fishes Balfour ('76, 

 pp. 393-402, and 78, pp. 15-24, PL II.) has figured and described 

 some interesting observations on the changes of the nucleus during seg- 

 mentation. The earlier stages of cleavage did not afford information 

 in this direction, but when the segments had become numerous, and 

 in diameter between 0.25™"" and 0.08™"^, it was observed, in sections of 

 specimens hardened in chromic acid, that the place of the nucleus was 

 occupied by a sharply defined figure having the shape of two cones placed 

 base to base, which stained as deeply as a nucleus. " From the apex 

 of each cone there diverge toward the base a series of excessively fine 

 striee. At the junction between the two cones is an irregular linear 

 series of small deeply stained granules, which form an apparent break 

 between the two. The line of this break is continued very indistinctly 

 beyond the edge of the figure on each side. From the apex of each 

 cone there diverge outwards into the protoplasm of the cell a series of 

 indistinct markings. They are rendered obscure by the presence of 

 yolk spherules, which completely surround the body just described, but 

 which are not arranged with any reference to these markings." 



The course of the markings (loc. cit., PI. II. Fig. 7 a), which evidently 

 correspond to asters, is quite unlike anything observed by others ; if not 

 fundamentally, at least in the extent of the deflection which the lines 

 suffer, causing, as it does, the most of them soon to take a course almost 

 parallel with the sides of their respective cones. I recall nothing similar 

 to this appearance, unless possibly the fibres which Strasburger ('76, 

 p. 45) describes as resulting from a resolution of the "mantle" of the 

 cask-like spindle into filaments in Spirogyra may be comparable to it. 

 In the present case, however, the "markings" do not form fibres continu- 

 ous from pole to pole. Balfour justly calls attention to the fact that the 

 striae of the cones are not to be confounded with the markings, for the 

 cones are quite as distinctly differentiated from the cell as nuclei are, 

 whereas the "markings" are merely structures in the general proto- 

 plasm of the cell. 



The end view of the spindle given by Balfour (Fig. 7 h) affords no idea 

 of the real distribution of the " linear series of granules," which repre- 

 sents the nuclear plate. The colored circular body in each cone, which 

 he once observed, — after a line indicating cell division had made its 

 appearance in the plane of the base of the cones, — may possibly rep- 

 resent the lateral zones of thickenings ; the drawing (Fig. 7 c), however, 

 leaves room for doubt. 



The important observation is made by Balfour that these conelike 



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