The cell in development and inheritance . a fact. Conklins (99) interesting observa-tions on the highly determinate cleavage of gasteropods iCrepidiila) ^ The convenient terms iiuictenninatc and determinate cleavage were suggested byConklin (98). 424 INHERITANCE AND DEVELOPMENT show that here the substance of the attraction-spheres is unequallydistributed, in a quite definite way, among the cleavage-cells, eachsphere of a daughter-cell being carried over bodily into one of thegranddaughter-cells (Fig. 192). We have here a substantial basis forthe conclusion that in cleavage of this type qualit
The cell in development and inheritance . a fact. Conklins (99) interesting observa-tions on the highly determinate cleavage of gasteropods iCrepidiila) ^ The convenient terms iiuictenninatc and determinate cleavage were suggested byConklin (98). 424 INHERITANCE AND DEVELOPMENT show that here the substance of the attraction-spheres is unequallydistributed, in a quite definite way, among the cleavage-cells, eachsphere of a daughter-cell being carried over bodily into one of thegranddaughter-cells (Fig. 192). We have here a substantial basis forthe conclusion that in cleavage of this type qualitative division of thecytoplasm may occur. It is important not to lose sight of the fact that development anddifferentiation do not in any proper sense first begin with the cleavageof the ovum, but long before this, during its ovarian history.^ Theprimary differentiations thus established in the cytoplasm form theimmediate conditions to which the later development must conform;and the difference between Amphioxns on the one hand, and the. Fig. 192. — Two successive stages in the third cleavage of the egg of Crepidula, seen from theupper pole. [Conklin.] In both figures the old spheres (dotted) lie at the upper pole of the embryo, and at the thirdcleavage they pass into the four respective cells of the first quartet of micromeres. The centro-somes are seen in the new spheres. snail or ctenophore on the other, simply means, I think, that theinitial differentiation is less extensive or less firmly established inthe one than in the other. The origin of the cytoplasmic differentiations existing at the be-ginning of cleavage has already been considered (p. 386). If theconclusions there reached be placed beside the above, we reach thefollowing conception. The primary determining cause of develop-ment lies in the nucleus, which operates by setting up a continuousseries of specific metabolic changes in the cytoplasm. This processbegins during ovarian growth, establishing the external f
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
