Handbook of field and general ornithology : a manual of the structure and classification of birds / with instructions for collecting and preserving specimens.

  • Coues, Elliott, 1842-1899.
Date:
1890
Catalogue details

Licence: Public Domain Mark

Credit: Handbook of field and general ornithology : a manual of the structure and classification of birds / with instructions for collecting and preserving specimens. Source: Wellcome Collection.

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    as I have observed before, it is like any other amoeba; the first step of development is probably a retrograde one; for if there ensues, when the spermatozoa melt into the ovum, the result affirmed for mammalian ova, the original germinal vesicle and germinal spot disappear, and the whole content of the ovum proper is simply a homogeneous mass of granular protoplasm. In this retrograde step, the organism, at the lowest possible round of the ladder of evolu- tion, is called a monerula. The germinal vesicle and spot, however, are speedily reconstructed, and the ovum looks precisely as it did before. But observe that the actual difference is enormous; for it Fig. 111.—Segmentation of the vitellus by discoiclal cleavage, diagrammatic, x about 10 times, after Haeckel. Only the “tread,” cicatricle, or germ-yelk (Figs. 109, &, 110, A) is represented, as no other part of the whole yelk-ball undergoes the process. A, separation into 2 ; B, into 4 ; C, into 16, by 8 radial and 1 concentric furrow; D, into many parts, by 16 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. now consists of the blended substance of the original ovum 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 entire future organism. In the former state it could reproduce nothing, not even itself; for it is the strange physiological law of a Dynamamoeha that it cannot reproduce like an ordinary cell, but must evolve an entire organism, like both of those two whose vital forces it concentrates, summarises, and embodies,—or nothing. The first change in the parent-cell is that by which it becomes broken up into a mass of cells, each of which is just like itself. This process is called segmentation of the vitellus; each one of the numerous resulting cells is called a cleavage-cell. The nucleus of