Identifier: cellindevelopmenwils2 (find matches)
Title: The cell in development and inheritance
Year: 1902 (1900s)
Authors: Wilson, Edmund B. (Edmund Beecher), 1856-1939
Subjects: Cell Physiological Phenomena Cells
Publisher: New York : Macmillan
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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-cell stage. C. Half eight-cell stage, fromisolated blastomere of the two-cell stage. D. Half twelve-cell stage succeeding. E. Two stagesin the cleavage of an isolated blastomere of the four-cell stage; above a one-fourth eight-cell stage,below a one-fourth sixteen-cell stage. able barrier to complete development. What determines the limita-tions of development in these various cases. They cannot be due tonuclear specification ; for in the ctenophore the fragment of an wiseg-mented Qgg, containing the normal egg-nucleus, gives rise to a defec-tive larva; and my experiments on Nereis show that even in a highly NATURE AND CAUSES OF DIFFERENTIATION 421 determinate cleavage, essentially like that of the snail, the nuclei maybe shifted about by pressure without altering the end-result. Neithercan they lie in the form of the dividing mass as some authors haveassumed; for in Cramptons experiments the half or quarter blasto-mere does not retain the form of a half or quarter sphere, but rounds
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Pig. 191. — Double embryos of frog developed from eggs inverted when in the two-cell stage. (O. SCHULTZE.) A. Twins with heads turned in opposite directions. B. Twins united back to back. C. Twinsunited by their ventral sides. D. Double-headed tadpole. off to a spheroid hke the Qgg. But if the limiting conditions lieneither in the nucleus nor in the form of the mass, we must seek themin the cytoplasm; and if we find here factors by which the tendencyof the part to develop into a whole may be, as it were, hemmed in, weshall reach a proximate explanation of the mosaic-like character ofcleavage shown in the forms under consideration, and the mosaic 422 INHERITANCE AND DEVELOPMENT theory of cytoplasmic localization will find a substantial if somewhatrestricted basis. That we are here approaching the true explanation is indicated bycertain very remarkable and interesting experiments on the frogs o-^^,which prove that each of the first two blastomeres may give rise eitherto a half-embryo

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