Text-book of comparative anatomy . The eggs here arise out of definitecells of the germinal epithelium, which grow more strongly and soon project beyondthe epithelium, with which, however, they remain for a time connected by a longstalk-like process. Through this stalk, in all probability, the nourishment of theegg by the epithelium takes place. Yolk granules, continually increasing in number,appear in the protoplasm of the egg. The nucleus becomes vesicular. The eggsecretes on its surface a yolk integument, which is broken through at the point THE EGG 29 where the egg joins the germinal epith
Text-book of comparative anatomy . The eggs here arise out of definitecells of the germinal epithelium, which grow more strongly and soon project beyondthe epithelium, with which, however, they remain for a time connected by a longstalk-like process. Through this stalk, in all probability, the nourishment of theegg by the epithelium takes place. Yolk granules, continually increasing in number,appear in the protoplasm of the egg. The nucleus becomes vesicular. The eggsecretes on its surface a yolk integument, which is broken through at the point THE EGG 29 where the egg joins the germinal epithelium, so that when the egg severs itself anopening, the micropyle, is left. The marine planaria offer a further example of a simple egg-formation (Fig. 24, D).These possess very numerous ovaries, whose nourishment is provided for by their. FIG. 24.—^!, 11, C, Three ovarian eggs of a mussel in different stages of development, afterFleming, m, Micropyle ; dh, yolk membrane. D, Ovary of a marine planarian with eggs indifferent stages of development; da, branch of the intestine or gastro-canal; l;l, germinal layer ;e, advanced egg. position between the branches of the intestine and close to its walls. In each ovarythere is a small germinal layer (kl) formed by a mass of small cells with nuclei andlittle protoplasm. Some of these cells grow andbecome eggs, numerous yolk granules forming inthem, and their nuclei changing into the charac-teristic germinal vesicle. Other cells remainsmall ; they take up a position between thestrongly growing egg cells, and so form a frame-work in the ovary, which is continued into theoviduct, As a type of a perfect meroblastic telolecithalegg with complicated envelope-formation we havethe birds egg (Figs. 25 and 26). The egg isfertilised within the mother body, and hasalready begun to develop
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
