Journal of morphology . and thenbegin to extend up between them {cf. Figs. 23 to 30, and Dia-gram II). The posterior median girdle-cell is the first to bedrawn out of the girdle, being pushed up between the products 396 WILSON. [Vol. VI. of c^ and d^ by the median division of X, as already described(Figs. 31, 32). Shortly after the bilateral division of the posterior micro-meres, c^ and d^, a similar bilateral unequal division takes placein the anterior pair a)- and b^ (Figs. 35 to 38, Diagram II). Asthis takes place, the micromeres move apart both from eachother and from the posterior microme
Journal of morphology . and thenbegin to extend up between them {cf. Figs. 23 to 30, and Dia-gram II). The posterior median girdle-cell is the first to bedrawn out of the girdle, being pushed up between the products 396 WILSON. [Vol. VI. of c^ and d^ by the median division of X, as already described(Figs. 31, 32). Shortly after the bilateral division of the posterior micro-meres, c^ and d^, a similar bilateral unequal division takes placein the anterior pair a)- and b^ (Figs. 35 to 38, Diagram II). Asthis takes place, the micromeres move apart both from eachother and from the posterior micromeres so as to form an ex-tremely regular and symmetrical cross, the centre of which isoccupied by the apical rosette. The three remaining intermedi-ate girdle-cells {ci^^, b^-^, c^-^) are at the same time drawn up intothe angles between the arms of the cross, and thus recede fromthe girdle, though they do not leave it entirely until somewhatlater. This will be rendered clear by the accompanying diagram(No. II). h_ ^ ^ t. Diagram shaded cells represent the intermediate girdle-cells and their progeny. Nearly at the same time, the entire set of girdle-cells dividein a very interesting symmetrical manner, which is illustratedby the diagram and is shown in Figs. 35, ^6, 39. In Diagram11, A {cf. Fig. 35) the direction of the division-spindles is shownby the arrows. Diagram II, B {cf. Fig. 39), shows the will be seen that each intermediate girdle-cell divides in asomewhat oblique plane, and is at the same time completelydrawn out of the girdle. The products of the trochoblastsmeanwhile divide in such wise that in each pair of cells onedivides horizontally and one vertically, in regular succession ina left-handed spiral around the girdle. Thus each of the origi- No. 3.] THE CELL-LINEAGE OF NEREIS. 397 nal trochoblasts gives rise to four cells, of which three remain inthe prototroch, and the fourth lies above it in the upper hemi-sphere of the larva. The twelve prototroc
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Keywords: ., bookauthorwistarin, bookcentury1800, bookdecade1890, bookyear1892
