Journal of morphology . esoblast-bands (dotted)extend straight forwards from the pigment-area {), at thecentre of the lower hemisphere, at right angles to the plane ofthe prototroch. The subsequent elongation of the body takesplace in the direction of the mesoblast-axis, m; parallel tothe mesoblast-bands; and the anus is formed in the centre ofthe space originally occupied by the pigment-area. The pig-ment-air a, therefore, represents the posterior extremity of the adultbody. Thus far the larva agrees precisely with the typical trocho-phore of Polygordins, Eupomatns, or Hydroides. As
Journal of morphology . esoblast-bands (dotted)extend straight forwards from the pigment-area {), at thecentre of the lower hemisphere, at right angles to the plane ofthe prototroch. The subsequent elongation of the body takesplace in the direction of the mesoblast-axis, m; parallel tothe mesoblast-bands; and the anus is formed in the centre ofthe space originally occupied by the pigment-area. The pig-ment-air a, therefore, represents the posterior extremity of the adultbody. Thus far the larva agrees precisely with the typical trocho-phore of Polygordins, Eupomatns, or Hydroides. As soon, how-ever, as the ventral plate is considered, we encounter anapparent contradiction. For the neural axis, n (which coincideswith the long axis of the ventral plate, of which the neuralplates form a part), is parallel to the prototroch, and hence atright angles to the mesoblast-axis. For reasons given at p. 436,it is impossible to doubt that the posterior teloblasts of Nereis No. 3.] THE CELL-LINEAGE OF NEREIS. 427. Diagram V. A. Early trochophore of Nereis from the left side; a^, d^, a^, a^-^, d^-^, t/^i,x^, regions derived from the corresponding micromeres; , frontal bodies; ,latero-dorsal region; , median dorsal region; ;«., mesoblast axis; n., neuralaxis; , cephalic neural plate; , polar cells; , pigment-area. The cephalicneural plates, the pigment-area, and the mesoblast bands are stippled; the ventralplate (product of X) shaded with lines. The dark body is the eye-spot. B. Corresponding view of later stage. Beginning of the shifting of the neuralaxis. Concrescence of the ventral neural plates. C. Diagram of the three-segmented larva (adult condition). Completion of theaxial shifting, coincidence of the neural and mesoblast axes. Shifting of the frontalbodies and of the ce]5halic neural plates. D. Young larva of Lopadorhynchus from the right side, before the shifting ofthe axes. 428 WILSON. [Vol. VI. {X) are the homologues of the neuro-nephrobl
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Keywords: ., bookauthorwistarin, bookcentury1800, bookdecade1890, bookyear1892
