Journal of Morphology, Volumen12,Temas1-2Wistar Institute of Anatomy and Biology ., 1897 |
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Términos y frases comunes
abdomen abnormal Amia amnion Anlage appear archoplasm Ascidians Baird blastomeres body cavity Botryllus branchial sac calotte cells cephalic lobes chelicerae chorion Clavelina cleavage coelom concrescence condition correlation depression disappeared dorsal duct ectoderm embryo endoderm endostyle environment evagination fact figures fission formation fourth fused fusion and degeneration ganglion gastrula Gmel Goodsiria grooves growth halves His's inner vesicle invagination lateral layer Limulus Linn marginal fold mass median fusion median line mesenchyme mesodermic area metamere nerve-cord nest neuromere nuclei oesophagus omphalomesenteric vein organs origin ovum pair of appendages peribranchial sacs pericardial pericardial cavity pericardium Perophora plate portion position posterior end produced pseudocoel region represents Ridgw Rocky Mts seen segments separated septum transversum shown in Fig side species specimens stage stolon structure surface views tail thickened thoracic appendages tion transverse transverse fission umbilical vesicle variation ventral vertebræ wall yolk zooids
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Página 245 - On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favorable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest...
Página 311 - Animals, pp. 659, 660. 1884. young, when about six inches long, make a famous bait for pickerel and pike. To use it, run the hook into the mouth right up through the centre of the head, through the brain, cast a hundred times, catch several fish, and at the end of three to six hours he will kick like a mule. Put one hundred in a rain-barrel and you can keep them all summer without change of water.
Página 245 - It seems clear that organic beings must be exposed during several generations to new conditions to cause any great amount of variation ; and that, when the organisation has once begun to vary, it generally continues varying for many generations.
Página 311 - The eggs hatch in eight and ten days, the parents remaining with the brood two or three weeks, if possible, but will leave them much sooner, if necessary, to save themselves. The young will not make any effort to escape to the lake until the next season, when, if an opening occurs, they come pouring out in countless numbers. At this time we take them by stretching the minnow seine across the opening and raising it when full. They are now from three to six inches long, fat and chubby.
Página 311 - I come now to mention a peculiar habit of this fish, no account of which I have ever seen. It is this : While the parent still remains with the young, if the family become suddenly alarmed, the capacious mouth of the old fish will open, and in rushes the entire host of little ones ; the ugly maw is at once closed, and off she rushes to a place of security, when again the little captives are set at liberty. If others are conversant with the above facts I shall be very glad, if not shall feel chagrined...
Página 245 - Gray have well remarked, by man; he can neither originate varieties nor prevent their occurrence; he can only preserve and accumulate such as do occur. Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues; but similar changes of conditions might and do occur under nature. Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions...
Página 245 - For though on the whole the offspring is like the parent or parents, its form is perhaps never identical with theirs, but generally differs from it perceptibly and sometimes materially. To this phenomenon, namely, the occurrence of differences between the structure, the instincts, or other elements which compose the mechanism of the offspring, and those which were proper to the parent, the name Variation has been given.
Página 313 - ... but occasionally too great aggressiveness on the part of the occupant or audacity of an intruder results in a regular battle. Whitman and Eycleshymer tell of one: Two males that claimed -a female were unwilling to yield, one to the other, and " a fierce battle for supremacy ensued " between them. " They approached from opposite sides of the nest and locked jaws in a most ferocious manner. Their struggles were so violent that a cloud of muddy water soon arose and obscured them from view.
Página 455 - In discussing this problem Parker ('96, p. 715) says, " Baur ('91' P- 33 0 i" a recent article, has shown very conclusively that, in place of one vertebra, two, or parts of two, may arise, the process evidently being a partial division of the material from which a single vertebra ordinarily arises. This process, which I should call the multiplication rather than the intercalation of vertebrae, may in some cases account for the increase of presacral elements, but I am not inclined to...
Página 334 - in the early stages of segmentation that the cleavage planes occurred in the normal way when the position of the egg was reversed." This is true not only for inverted eggs, but for eggs placed in any position whatsoever. It seems to follow that gravity can have no directing influence on the cleavage. In order to ascertain whether there is any constant relation of the embryonic axis to either of the first two cleavage planes, eggs were fixed in given positions by weighting, as before mentioned, and...