File:Journal of morphology (1892) (14779866741).jpg

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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, the larval axes are at right angles to those ofthe adult. Thus, (i) the posterior extremity, (2) the ventral region, and (3) theanterior region of the embryo and early larva become in v. Wistinghausens termi-nology respectively (i) dorsal, (2) posterior, and (3) ventral. I regret this confusionof terminology, since I am perfectly in accord with v. Wistinghausen in regard to thefacts. I wish, however, expressly to disclaim any intention of attaching a profoundmorphological significance to the terminology employed; for, as will be shown here-after, the axes of the young embryo cannot be very accurately defined. The generalaxial shifting that takes place in the metamorphosis is, however, not ideal, but real —it is an actual transference of material; and if this shifting is not recognized, theterminology of the early stages becomes in the highest degree confusing and contra-dictory. My orientation is, therefore, adopted for purely practical reasons, i.e. forconvenience of description.
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No. 3.) THE CELL-LINEAGE OF NEREIS. 383 A general view of the cleavage may be obtained from theaccompanying diagram or cytogenetic tree (p. 382), which repre-sents accurately the genetic relations of the blastomeres up tothe complete establishment of bilaterality. The number of blas-tomeres in the successive stages is given in the upper line ; thesuccession of the divisions is shown by the vertical columns.The record is without a gap up to the fifty-eight-celled stage.Beyond this point the development of the embryo as a wholecannot be fully represented in the diagram, on account ofincreasing variations in the order of division of the individualcells. Special diagrams showing the history of some of theindividual protoblasts will be given further on. III. Spiral Period of Cleavage (l to 38 blastomeres). I. The First Cleavage (Figs. 2* to 3). The first cleavage takes place about seventy-five minutes afterfertilization. The plane of division passes slightly to one sideof the polar cells,

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