MUSEUM OF COMPARATIVE ZOOLOGY. 137 



characteristic of the resting nucleus as of the mitotic nucleus, was 

 postulated by Rabl ('85, p. 323) from a careful study of the chromatic 

 network, in the " skein stage " of mitosis. In a recent paper ('89, pp. 23, 

 24), the same writer states that the "polar depression," usually visible 

 in young daughter nuclei, persists much longer than usual in the epi- 

 thelial nuclei of the Triton ; so that for these mitotically dividing nu- 

 clei it is highly probable that polar differentiation is always present in 

 the resting state. Carnoy ('85) has shown that, in the resting nuclei 

 of the testicular cells of certain Arachnids, the chromatic filaments are 

 distinctly arranged with reference to a definite axis (Planche V. Figs. 

 165-169), and Van Gehuchten ('89) has found the same in glandular 

 cells of a Dipterous insect, Ptycoptera contaminata. 



It is obvious that the discovery of an " organic axis," as Van Gehuch- 

 ten calls it, in amitotically dividing nuclei is more difficult, for here there 

 is no polar depression or longitudinal arrangement of chromatic fila- 

 ments to indicate its direction in the resting nucleus. It is usual for 

 each division of the nuclei of the serosa to take place at right angles, or 

 nearly so, to the plane of the previous division. This is well seen in 

 many multinuclear cells, where one or both pairs of nuclei lie trans- 

 versely in the cell, and therefore at right angles, or nearly so, to the 

 direction of the first division (see cells 2 and 5, Fig. 14). In other 

 cases, however, two consecutive divisions take place in the same direc- 

 tion (Fig. 14, cell 1). It occurred to me that possibly tiiere was an 

 organic axis in the nuclei of the serosa which in some cases exerted a 

 controlling influence upon the direction in which division took place, 

 but which in most instances was counteracted by influences resident in 

 the cytoplasm. Transverse divisions of the nucleus (Fig. 12) could then 

 be accounted for by assuming that the influence of the organic axis is 

 dominant in these cases, while oblique divisions would be explainable on 

 the ground that neither influence was predominant, but that both acted 

 with about equal force in directions at right angles to each other. A 

 question of interest in this connection is, whether, when the cytoplasmic 

 influence is dominant, and tends to make the nucleus divide in a plane 

 parallel to its organic axis, division actually does take place in that 

 direction. If such were the case, an organic axis would be a fact of 

 slight morphological importance, and the longitudinal arrangement of 

 chromatin, which takes place in the earlier stages of constriction (Figs. 

 4, 6, 7), might occur in any direction, without reference to an organic 

 axis. If, on the contrary, it were necessary that the longitudinal fila- 

 ments should be arranged parallel to the organic axis, in order that 



