File:The cell in development and inheritance (1896) (19965093244).jpg

Title: The cell in development and inheritance
Identifier: cellindevelopmen1896wils (find matches)
Year: 1896 (1890s)
Authors: Wilson, Edmund B. (Edmund Beecher), 1856-1939
Subjects: Cells
Publisher: New York : The Macmillan company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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GEOlMETRICAL RELATIONS OF CLEAVAGE-FORMS 71 extent of the inequality varies in like manner. Taking the third cleavage as a type, we may trace every transition from an equal divi- sion (echinoderms, Polygordius), through forms in which it is but slightly marked (Amphioxus, frog), those in which it is conspicuous (^Nereis, Lymjicea, Polycladcs, Pctroviyzoii, etc.), to forms such as Clcp- siue, where the cells of the upper quartet are so minute as to appear like mere buds from the four large lower cells (Fig. 123). At the
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Fig. 122. — Cleavage of Nereis. An example of a spiral cleavage, unequal from the beginning and of a marked mosaic-like character. A. Two-cell stage (the circles are oil-drops). B. Four-cell stage; the second cleavage-plane passes through the future median plane. C. The same from the right side. D. Eight-cell stage. E. Sixteen cells ; from the cells marked t arises the prototroch or larval ciliated belt, from A' the ventral nerve-cord and other structures, from D the mesoblast-bands, the germ-cells, and a part of the alimentary canal. F. Twenty-nine-cell stage, from the right side; /. girdle of prototrochal cells which give rise to the ciliated belt. extreme of the series we reach the partial or meroblastic cleavage, such as occurs in the cephalopods, in many fishes, and in birds and reptiles. Here the lower hemisphere of the egg does not divide at all, or only at a late period, segmentation being confined to a disc- like region or blastoderm at one pole of the Qgg (Fig. 124). Very interesting is the case of the tcloblasts qx pole-cclls character- istic of the development of many annelids and mollusks and found in some arthropods. These remarkable cells are large blastomeres, set aside early in the development, which bud forth smaller cells in reg-

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