File:Journal of morphology (1892) (14782665912).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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lastoporetypically proceeds forwards from the posterior lip, in the direc-tion of the arrow, its anterior portion usually persisting as themouth, or marking its position. During, or sometimes before,the process of closure the mesoblast-cells pass into the cleavage-cavity, where they ultimately give rise to the mesoblast-bands. Among the annelids the axial relations of the mesoblast-bands(and also of the neural bands) with reference to the blastoporeare of two types, (i) In the first type, typically 7epresented by No. 3.) THE CELL-LINEAGE OF NEREIS. 429 Liimbricits (Diagram VI, E) and in a modified fovm by Rhyn-cJielmis and the Hiriidinea, the primary mesoblasts remain in theiroriginal position, the mesoblast-bands are parallel to the originallips of the blastopore, and the embryo7iic mesob last-axis persistsin the adult. The same is true of the neural axis ; for theneural bands are parallel to the mesoblast-bands, forming withthe latter the principal basis of the so-called germ-bands.
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Diagram VI. A. Early annelidan gastrula. The mesoblast is shaded. B. Horizontal type of the foetal form (^Lumbrictis). C. Vertical type of the larval form (Polyg07-dins, etc.). For the evidence on which this statement rests I refer to mypaper on Lnmbricus (No. 29) and to the well-known researchesof Whitman and Vejdovsky. (2) In the second type, represented by the PolychcBta in general(Diagram VI, C), the primary mesoblasts or their eqinvalents aresooner or later carried downwards until they lie approximately atthe centre of the lower hemisphere. The mesoblast-bands ai-etherefore at right angles to the lips of the blastopore; i.e. the 430 WILSON. (Vol. VI. mesoblast axis is rotated through 90°. The same is true of theneural axis. The truth of this statement becomes at once apparent onconsidering the development of LopadorhyncJius, where the ro-tation of the axis is effected very late in the development. Inthe young lava (Diagram V, D) the mesoblastic and neuralbands are horizontal and

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