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Weismann's hypothesis is also …

CRITIQUE OF THE ROUX-WEISMANN THEORY From a logical point of view the Roux-Weismann theory is unassailable. Its fundamental weakness is its quasi-metaphysical character, which indeed almost places it outside the sphere of legitimate scientific hypothesis. Not a single visible phenomenon of cell-division gives even a remote suggestion of qualitative division. All the facts, on the contrary, indicate that the division of the chromatin is carried out with the most exact equality. The theory of qualitative division was suggested by a totally different order of phenomena, and is an explanation constructed ad hoc. Roux, it is true, was led to the hypothesis through an examination of mitosis ; but it is safe to say that he would never have maintained in the same breath that mitosis is expressly designed for quantitative and also for qualitative division, had he fixed his attention on the actual phenomena of mitosis alone. The hypothesis is in fact as complete an a priori assumption as any that the history of scholasticism can show, and every fact opposed to it has been met by equally baseless subsidiary hypotheses, which, like their principal, relate to matters beyond the reach of observation.

01/05/2012 · As predicted by the Weismann hypothesis, ..

CRITIQUE OF THE ROUX-WEISMANN THEORY From a logical point of view the Roux-Weismann theory is unassailable. Its fundamental weakness is its quasi-metaphysical character, which indeed almost places it outside the sphere of legitimate scientific hypothesis. Not a single visible phenomenon of cell-division gives even a remote suggestion of qualitative division. All the facts, on the contrary, indicate that the division of the chromatin is carried out with the most exact equality. The theory of qualitative division was suggested by a totally different order of phenomena, and is an explanation constructed ad hoc. Roux, it is true, was led to the hypothesis through an examination of mitosis ; but it is safe to say that he would never have maintained in the same breath that mitosis is expressly designed for quantitative and also for qualitative division, had he fixed his attention on the actual phenomena of mitosis alone. The hypothesis is in fact as complete an a priori assumption as any that the history of scholasticism can show, and every fact opposed to it has been met by equally baseless subsidiary hypotheses, which, like their principal, relate to matters beyond the reach of observation.

Mutation Accumulation Theory of Aging

Are the data they obtained consistent with Weismann’s hypothesis? Explain your thinking. 4. What conclusions can you make based upon the data?

not, according to modern biology, cease to be a germ-cell in any stage of the complicated process. The original germ-cell splits up into two: one half gradually develops itself into the body of a plant or animal—this is the somatic cell; the other half remains encased within the body practically unchanged, and is transmitted in the process of reproduction to the offspring—that is, the germ-plasm. Now, this germ-plasm is unbroken through the whole line of propagation. This is Weismann's doctrine of 'continuity of the germ-plasm,' which has been widely accepted, though it is but an hypothesis."

Such an hypothesis cannot be actually overturned by an appeal to fact. When, however, we make such an appeal, the improbability of the hypothesis becomes so great that it loses all semblance of reality. It is rather remarkable that Roux himself led the way in this direction. In the course of his observations on the development of a halfembryo from one of the blastomeres of the two-cell stage he determined the significant fact that the half-embryo afterwards regenerated the missing half, and gave rise to a complete embryo. Essentially the same result was reached by later observers, both in the frog (Endres, Walter, Morgan) and in a number of other animals, with the important addition that the half-formation is sometimes characteristic of only the earliest stages and may be entirely suppressed. In 1891 Driesch was able to follow out the development of isolated blastomeres of sea-urchin eggs separated by shaking to pieces the twocell and four-cell stages. Blastomeres thus isolated segment as if still forming part of an entire larva, and give rise to a half- (or quarblastula (Fig. 132). The opening soon closes, however, to form a small complete blastula, and the resulting gastrula and Pluteus larva is a perfectly formed dwarf of only half (or quarter) the normal size. Incompletely separated blastomeres gave rise to double embryos like the Siamese twins. Shortly afterwards the writer obtained similar result in the case of Amphioxus, but here the isolated blastomere segments from the beginning like an entire ovum of diminished size (Figs. 133, 124). A like result has since been reached by Morgan ('95, 2) in the teleost fishes, by Herlitzka ('95) in Triton, and by Zoja ('95, I) in the medusae. The last-named experimenter was able to obtain perfect embryos not only from blastomeres of the two-cell and fourcell stages, but from eight-cell and even from sixteen-cell stages, the dwarfs in the last case being but -1/16 the normal size ! These experiments gave a fatal blow to the entire Roux-Weismann theory ; for the results showed that the cleavage of the ovum does not in these cases sunder different materials, either nuclear or cytoplasmic, but only splits it up into a number of similar parts, each of which may give rise to an entire body of diminished size.

Evolutionary theories of aging and longevity

Tag: 3. Are the data they obtained consistent with Weismann’s hypothesis? Explain your thinking. Can you design a way to test the hypothesis that asexual reproduction

Lamarckism: History of Lamarckism, an early theory of organic evolution.
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