Buckmaster's Elements of animal physiology / by John Angell.

  • Angell, John
Date:
1866
Catalogue details

Licence: Public Domain Mark

Credit: Buckmaster's Elements of animal physiology / by John Angell. Source: Wellcome Collection.

    361/384 page 349
    during incubation; they only take place within certain temperatures, approximating to that of the body. The following is a general description of the changes which take place in the ova (or eggs) of the mammalia during the development of the embryo:— The omim of a mammifer consists of an external membrane, the zona pellucida, or vitelline membrane, which encloses a' granular yolk, the germ yolk ; in the centre of the yolk is the germinal vesicle (the germ cell), containing the germinal spot. When the orvum becomes fertilized by the influence of the sperm cell—I, the sperm cell germinal vesicle dissolve in the yolk ; 2, the granular commences a process of segmentation—first separating into two parts, these into four the latter into eight, and so on until the whole is reduced to a mulberry-like mass of distinct nucleated cells; 3, this mulberry-like mass of cells be- comes differentiated, forming into connective and muscular fibre, cartilage, bone, membrane, vessels, &c. ; [a) an elevation forms on the surface, which gradually divides into a groove (the primi- tive groove)-, {b) the lateral boundaries of this groove extend upwards, forming the tube for enclosing the brain and spinal cord, and downwards, forming a tube for the alimentary canal, heart, and bloodvessels ; (r) processes forming rudimentary limbs are given off from the sides. In the earlier stages of development the embryos of all animals are very similar; in the later stages the parts acquire their specialities; and lastly, the embryo is converted into the form of the parent from which'it originated. Changes of Animal Existence. 1st stage, A minute particle of nitrogenous matter. 2nd ,, Develops into an embryo having the special type and construction of the parent. 3rd ,, Is bom, works, wastes, and renovates. 4 th ,, Dies. 5th ,, Decomposes and returns to matter. The offspring inherits the general appearance, form, and qualities, mental and physical, of its parent. Whatever raises the type of the parent improves its
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