A Laboratory text-book of embryology

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P. Blakiston's Son, 1910 - 402 pages
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Page 20 - ... but slightly from the ratio established when the embryo begins to take food from outside. It is only for convenience that I express this law in this precise form; in reality, about it our knowledge is scanty and our conceptions vague. According to a geometrical principle, when the bulk of a body bounded by a simple surface increases, the surface enlarges less than the mass — in the simplest case of a cube, the surface increases as the square, the mass as the cube, of the diameter. If in a cube...
Page 29 - ... gemmules. They are supposed to be transmitted from the parents to the offspring, and are generally developed in the generation which immediately succeeds, but are often transmitted in a dormant state during many generations and are then developed. Their development is supposed to depend on their union with other partially developed cells or gemmules which precede them in the regular course of growth.
Page 29 - It is almost universally admitted that cells, or the units of the body, propagate themselves by self-division or proliferation, retaining the same nature, and ultimately becoming converted into the various tissues and substances of the body. But besides this means of increase I assume that cells, before their conversion into completely passive or ' formed material,' throw off minute granules or atoms, which circulate freely throughout the system,.
Page 12 - ... that the earliest cells (segmentation spheres, blastomeres, or cleavage cells, as they are variously called) produced by the ovum preserve the undifferentiated qualities of the parent egg, and may develop in one way or another according to circumstances. The egg of a sea-urchin divides into two cells, each of which multiplies and normally gives rise to half of the body of the animal. By somewhat violent shaking the two cells may be artificially separated; each cell may then develop into a complete...
Page 24 - ... special factors involved here is well expressed by Minot (:03, pp. 38-39). '• The reason for the unequal growth is unknown. We have not even an hypothesis to offer as to why one group of cells multiplies or expands faster than another group of apparently similar cells close by in the same germ layer. It is no real explanation to say that it is the result of heredity, for that leaves us as completely in the dark as ever as to the physiological factors at work in the developing individual.
Page 11 - ... operative in the lower animals which have neither intestines or arteries and in man ; and he ascribes senescence to the increase and differentiation of cytoplasm as compared with nucleoplasm. In 1901 he proposed "the new term cytomorphosis to designate comprehensively all the structural alterations which cells, or successive generations of cells may undergo from the earliest undifferentiated stage to their final destruction.
Page 62 - Ex. 2. A particle is acted upon by forces represented by P, 2P, 3/J3P, and 4P; the angles between the first and second, the second and third, and the third and fourth are 60, 90, and 150 respectively.
Page 94 - Balfonr (Works, Vol. i) and others that the blood corpuscles consist only of nuclei. By following the development along further we find that the protoplasm enlarges for several days, and that during the same time there is a progressive diminution in size of the nucleus, which, however, is completed before the layer of protoplasm reaches its ultimate size. The nucleus is at first granular, and its nucleolus, or nucleoli, stand out clearly; as the nucleolus shrinks it becomes round and is colored darkly...
Page 363 - Numerous valve-like apertures are observed upon all parts of the surface. They are the orifices of the veins which have been torn off from the uterus. A probe passed into any one of these, after taking an oblique direction, enters at once into the placental substance. Small arteries, about half an inch in length, are also everywhere observed embedded in this layer. After making several sharp spiral turns, they likewise suddenly open into the placenta"; and on page 719 he adds: "These venous orifices...
Page 12 - Born, who further discovered that the segmentation nucleus always rises toward the upper side of the egg, and that the position of the nucleus determines which part of the ovum shall become the dorsal side of the embryo.

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