The cell in development and inheritance . AmpJii-oxus and the hydromasae in which a perfect whole development usually takes place from the beginning, though it is a very interest-ing fact that the isolated blastomeres of AmpJiioxus sometimes show,in the early stages of cleavage, peculiarities of development that recalltheir behaviour when forming part of an entire embryo.^ We see throughout this series an effort, as it were, on the part ofthe isolated blastomere to assume the mode of development character-istic of a complete Qgg, but one that is striving against conditions that 1 This is not i
The cell in development and inheritance . AmpJii-oxus and the hydromasae in which a perfect whole development usually takes place from the beginning, though it is a very interest-ing fact that the isolated blastomeres of AmpJiioxus sometimes show,in the early stages of cleavage, peculiarities of development that recalltheir behaviour when forming part of an entire embryo.^ We see throughout this series an effort, as it were, on the part ofthe isolated blastomere to assume the mode of development character-istic of a complete Qgg, but one that is striving against conditions that 1 This is not invariably the case, as described beyond. 2 Cf. Wilson, 93, pp. 590, 60S. 420 INHERITANCE AND DEVELOPMENT tend to confine its operations to the role it would have played if stillforming part of an entire developing o^gg. In Aniphioxiis or Clytiathis tendency is successful almost from the beginning. In otherforms the limiting conditions are only overcome at a later period,while in the ctenophore or snail they seem to afford an insurmount-. C D ^—^ E Fig. 190. — Partial development of isolated blastomeres of the gasteropod egg, Ilyanassa.[Crampton.] A. Normal eight-cell stage. B. Normal sixteen-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. Neithe
Size: 1584px × 1577px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
