. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. INTRODUCTION 3 the germinal vesicle, and one or more germinal spots. The outer covering of the ovum is spoken of as the vitelline membrane. Since the ovum in its primitive form as above described repre- sents a single cell, we may speak of the vitellus ^ as the protoplasm of the egg-cell, the germinal vesicle as its nucleus, and the germinal spot as its nucleolus. The cell-nucleus is enclosed by a delicate nuclear membrane, and is made up of two constituentsâthe spongioplasm or chromatin, and the hi/aloplasm or achroma
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. INTRODUCTION 3 the germinal vesicle, and one or more germinal spots. The outer covering of the ovum is spoken of as the vitelline membrane. Since the ovum in its primitive form as above described repre- sents a single cell, we may speak of the vitellus ^ as the protoplasm of the egg-cell, the germinal vesicle as its nucleus, and the germinal spot as its nucleolus. The cell-nucleus is enclosed by a delicate nuclear membrane, and is made up of two constituentsâthe spongioplasm or chromatin, and the hi/aloplasm or achromatin. One or two small particles, the centrosomes, are also present in the cell-body, and take an important part in the process of cell- division. An outer limiting membrane, corresponding to the vitelline membrane, is not an integral part of the cell, but may be dilieren- tiated as a hardening of the peripheral protoplasm. In sexual reproduction, such as / I ^ Jâ\-iLji occurs in all Vertebrates, the fusion of -»- the sperm-cell, containing the genera- tive substance of the male, with the ovum, is an absolute necessity for the development of the latter. Fic. i.âDiagram of thk But before this can occur, certain Unimpregsatbd Ovum. changes take place in the ovum, which j)^ vitellus; KB, germinal are known as maturation. This con- vesicle; KF, germinal spot, sists of a twice-repeated process of cell- division (Jcaryokinesis) similar to that which occurs in tissue- cells, except that the resulting daughter-cells are of different sizes, two small nucleated polar-cells (Fig. 2) being successively thrown off from the larger ovum, the portion of the original nucleus remaining in the ovum being known as the "â female promicleus" A sperm-cell {spermatozoon) then makes its way into the ovum, and its nucleus (the male pronucleus) unites with the female pronucleus to form the segmentation nucleus. This process, which is known as impregnation or fertilisation, thus consists i
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
