File:Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl (sic) (1893) (20354053046).jpg

Title: Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl (sic)
Identifier: biologicallectur1893mari (find matches)
Year: 1893 (1890s)
Authors: Marine Biological Laboratory (Woods Hole, Mass. )
Subjects: Biology
Publisher: Boston, Ginn & Co
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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THE MOSAIC THEORY OF DEVELOPMENT \\ ments of truth in the mosaic hypothesis ; and I will attempt to indicate this modification by a comparison between Amphi- oxns and Nereis. In the case of AmpJiioxus we have the clear- est evidence that differentiation is, in a measure, dependent upon the relation of the cell to the whole of which it forms a part. The first visible differentiation in this case is at the third cleavage, which consists in an unequal division of each of the four blastomeres, so as to give rise to four micromeres and four macromeres, the former giving rise to ectoblast only, while the latter give rise to entoblast and mesoblast as well (Diagram I). If, however, the blastomeres of the 4-celled stage be separated (shaken apart) the course of events is entirely changed ; for in this case each divides equally, not unequally, and ultimately gives rise to a complete quarter-sized Uiagrvm I dwarf, instead of one-quarter of a normal embryo, as it would have done under ordinary cir- cumstances. The character of the fourth cleavage is here directly or indirectly determined in each cell by the relation of the cell to its fellows ; and if this is true of any one stage of the ontogeny, a very strong presumption is created that it is true of all — that, in the process of progressive differentiation occurring in the course of every animal on- togeny, the character of each step is determined by the condition of the entire organism. The ontogeny is, in other words, a connected series of interactions between the various parts of the embryo, in which each step estab- lishes new relations, through which the following step is determined. The character of the series, as a whole, depends upon the first step, and this in turn upon the constitution of the original ovum. In AinpJiioxtis differentiation proceeds slowly, the earlier blastomeres show no appreciable divergence, and the first stages show no trace of a mosaic work. In Nereis, on the other hand, a mosaic-like character appears from the begin- ning, because of the inequality of the first cleavage, which conditions the entire subsequent development through the

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