. The cell in development and inheritance. Cells. GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 369 required to show that this purely mechanical factor, though doubtless real, must be subordinate to some other. This is strikingly shown, for example, in the development of annelids and mollusks, where the spiral cleavage, strictly maintained during the earlier stages, finally gives way more or less completely to a bilateral type of division in which the rule of minimal surface-contact is often violated. We see here a tendency operating directly against, and finally overcoming,. E F Fig. 171. — Cleavag
. The cell in development and inheritance. Cells. GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 369 required to show that this purely mechanical factor, though doubtless real, must be subordinate to some other. This is strikingly shown, for example, in the development of annelids and mollusks, where the spiral cleavage, strictly maintained during the earlier stages, finally gives way more or less completely to a bilateral type of division in which the rule of minimal surface-contact is often violated. We see here a tendency operating directly against, and finally overcoming,. E F Fig. 171. — Cleavage of Nereis. An example of a spiral cleavage, unequal from the beginning and of a marked determinate character. ^.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 ^Y the ventral nerve-cord and other structures, from D the mcsoblast-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. the mechanical factor which predominates in tlie earlier stages; and in some cases, in the o.^^ of Clavelina (Fig. 177) and other tuni- cates, this tendency predominates from the beginning. In both these cases this " tendency " is obviously related to the growth-process to which the future bilateral embryo will owe its form ; ^ and every attempt to explain the position of the cells and the direction of cleav- age must reckon with the morphogenic process taken as a whole. The blastomere is not merely a cell dividing under the stress of rude * Cf. Wilson ('92, p. 444). 2B. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustr
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectcells, bookyear1911
