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4-celled stage acquired characters adaptation adult Amphioxus animals annelids appears archoplasm Baer's law become Befruchtung bilateral blasto blastomeres blastopore blastula body breed causes of variation centrosome changes chromatin chromosomes cleavage constitution corresponding Darwin differentiation dimorphism direction disappear divide division double Echinus eggs Eikern embryo evolution existence external fact factors female fertilized fibres figures gastrula germ-plasm give Hertwig horns idioplasm individual influence inheritance insects intra-selection Kern larvae less Limax limbs lower pole male ment metameres micromeres modification natural selection Nereis Noctiluca normal nuclear nucleus Obelia observed occur organs origin ovaries ovum physiological plants Plate Polyspermie position primary constituents produced protoplasm quartet radial regeneration reproductive cells result rings Roux seen segmentation segmentation nucleus sexual shown side similar somatic cells species Spermakern spermatozoa spermatozoon spindle spiral spiral cleavage spore-formation stimulus structure take place Theilung theory tion tissue unsegmented tract Weismann workers yolk
Page 18 - I had not formerly sufficiently considered the existence of many structures which appear to be, as far as we can judge, neither beneficial nor injurious; and this I believe to be one of the greatest oversights as yet detected in my work.
Page 40 - ... no shadow of reason can be assigned for the belief that variations, alike in nature and the result of the same general laws, which have been the groundwork through natural selection of the formation of the most perfectly adapted animals in the world, man included, were intentionally and specially guided.
Page 51 - Dog: — the yelk of that egg always undergoes division, or segmentation as it is often called: the ultimate products of that segmentation constitute the building materials for the body of the young animal; and this is built up round a primitive groove, in the floor of which a notochord is developed.
Page 16 - Animal Life" (1877) we have the best systematized effort to bring together the direct causes of variation, and no one who has read through its pages can doubt the direct modifying influences of nutrition, light, temperature, water at rest and in motion, atmosphere still or in motion, etc., or question his conclusion that no power which is able to act only as a selective and not as a transforming influence can ever be exclusively put forth as a causa efficiens of the phenomena.
Page 38 - It has always pleased me to exalt plants in the scale of organized beings, and I have therefore felt an additional pleasure in showing how many and what well adapted movements the tip of a root possesses.
Page 51 - In my possession are two little embryos in spirit, whose names I have omitted to attach, and at present I am quite unable to say to what class they belong. They may be lizards, or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals.
Page 20 - ALONG with that inadequacy of natural selection to explain changes of structure which do not aid life in important ways, alleged in § 166 of " The Principles of Biology," a further inadequacy was alleged. It was contended that the relative powers of co-operative parts cannot be adjusted solely by survival of the fittest ; and especially where the parts are numerous and the cooperation complex.
Page 39 - ... Weismann's hypothesis. They show that there is none of the alleged independence of the reproductive cells, but that the two sets of cells are in close communion. They prove that while the reproductive cells multiply and arrange themselves during the evolution of the embryo, some of their germ-plasm passes into the mass of somatic cells constituting the parental body, and becomes a permanent component of it.
Page 33 - He argues that so long as the parent form remained most in harmony with the surrounding conditions it would maintain in the struggle for existence its characteristics against all tendency to vary in its offspring ; which is equivalent to saying that it will remain unchanged so long as the environment remains the same. He then shows that in organisms in which the reproductive period covers many years, accelerated development by primogeniture, ie, as between the first born and the last born of any...
Page 61 - A peculiar and, it seems to be, an inexplicable fact is, that previous pregnancies have an influence upon offspring. This is well known to breeders of animals. If pure-blooded mares or bitches have been once covered by an inferior male, in subsequent fecundations the young are likely to partake of the character of the first male, even if they be afterwards bred with males of unimpeachable pedigree.