Identifier: keytonorthameric00coues (find matches)
Title: Key to North American birds. Containing a concise account of every species of living and fossil bird at present known from the continent north of the Mexican and United States boundary, inclusive of Greenland and Lower California, with which are incorporated General ornithology: an outline of the structure and classification of birds; and Field ornithology, a manual of collecting, preparing, and preserving birds
Year: 1896 (1890s)
Authors: Coues, Elliott, 1842-1899
Subjects: Birds Birds
Publisher: Boston, Estes and Lauriat
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Smithsonian Libraries

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ly as it did be-fore. But observethat the actual dif-ference is enormous;for it now consistsof the blended sub-stance of the original o\yim and of the spermatozoa ; and in this duplex or bisexed state,before any further step is taken, the creature is called a cytula, — the parent cell of the entirefuture organism. In the former state it could reproduce nothing, not even itself; for it is thestrange physiological law of a Di/namamaeba that it cannot reproduce like an ordinary cell,but must evolve an entire organism, like both of those two whose vital forces it concentrates,summarizes, and embodies, — or nothing. The first change in the parent-cell is that by which it becomes broken up into a mass ofcells, each of which is just like itself. This process is called segmentation of the vitellus^ eachone of the numerous resulting cells is called a cleavage-cell. The nucleus of the parent-celldivides into two; each attracts its half of the yelk ; the halves fuiTow apart and there are now
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Pig. Ill -Segmentation of the vitellus by rlisciidal cleavage, diagrammatic, x about10 times, after Haeckel. Only the ■•tread. ciiatricle. or germ-yelk (figs. 109,6,110, A) isrepresented, as no other part of the whole yelk-ball undergoes the process. A, separationinto 2; B, into 4; C, into 16, by 8 ra, into many parts, by16 radial .and about 4 concentric furrows; E, 64 radial and about 6 concentric furrows;F, the whole tread broken up into a mulberry-mass (morula) of cells. THE ANATOMY OF BIRDS.—OOLOGY. 225 two cleavage-cells in place of the one parent-cell. A furrow at right angles to the first, andredivi^sion of the nuclei, results in/ottr cleavage-cells. Radiating furrows intermediate to thefirst two bisect the four cells, and would render eight cells, were not these simultaneouslydoubled by a circular fuiTow which cleaves each, with the result of sixteen cleavage-cells. Sothe subdivision goes on until the parent-cell becomes a mass of c

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