Identifier: journalofmorphol1892wist (find matches)
Title: Journal of morphology
Year: 1892 (1890s)
Authors: Wistar Institute of Anatomy and Biology
Subjects:
Publisher: Philadelphia, Wistar Institute of Anatomy and Biology
Contributing Library: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library
Digitizing Sponsor: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library

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esoblast-bands (dotted)extend straight forwards from the pigment-area (p.a.), 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; i.e. 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
Text Appearing After Image:
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; f.li, frontal bodies; l.d.,latero-dorsal region; m.d., median dorsal region; ;«., mesoblast axis; n., neuralaxis; n.p., cephalic neural plate; p.c, polar cells; p.a., 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-nephroblasts of the Hiru-dinea a

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