. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. INTRODUCTION 3 (Fig. I), consisting of the vitellus, in the interior of which is the germinal vesicle, enclosing one or more germinal spots: the membrane which covers the vitellus is spoken of as the vitelline membrane. Since the ovum corresponds to 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 vitellus, however, consists of two different sub- stances — protoplasm and deuteroplasm (yolk} — in varying propor-
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. INTRODUCTION 3 (Fig. I), consisting of the vitellus, in the interior of which is the germinal vesicle, enclosing one or more germinal spots: the membrane which covers the vitellus is spoken of as the vitelline membrane. Since the ovum corresponds to 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 vitellus, however, consists of two different sub- stances — protoplasm and deuteroplasm (yolk} — in varying propor- tion and relative distribution in different animals. The nucleus is enclosed by a delicate nuclear membrane, and is made up of two chief constituents — the spongioplasm or chromatin, and the hyaloplasm or achromatin. A small particle, the ccntrosome, is also present in the protoplasm of the cell, and takes an important part in the process of cell-division. An outer limiting membrane, corresponding to the vitelline membrane, is not an in- tegral part of the cell, but may be differentiated from the peripheral pro-. toplasm ^IC{- 1- — DIAGRAM OF THE In the process of sexual reproduc- tion, which occurs in all Vertebrates, A vitellus; KB, germinal the fusion with the ovum of the vesicle ; KF> Serminal sPot' sperm-cell or spermatozoon, con- taining the generative substance of the male, is an absolute necessity for the development of the former. Before this fusion can occur, certain changes take place in the ovum which constitute what is known as its maturation, the essential result of which is a reduction in mass of the chromatin in the germinal vesicle. The ovum undergoes a twice-repeated process of cell division (karyokinesis or mitosis) similar to that which occurs in tissue-cells, except that the resulting daughter- cells, in addition to the reduction in their chromatin, are of different sizes, two small evanescent polar-cells (Fig. 2) being suc- cessively thrown off
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