. The cell in development and inheritance. Cells. 146 FERTILIZATION OF THE OVUM enters (Fig. 74). Thus, the starfish Q^g, according to Fol, pos- sesses before fertilization a pecuhar protoplasmic " attraction-cone " to which the head of the spermatozoon becomes attached, and through which it enters the Qgg. In some of the hydromedusse, on the other hand, the entrance point is marked by a funnel-shaped depression at the egg-periphery (Metschnikoff). When no preformed attraction- cone is present, an " entrance-cone " is sometimes formed by a rush of protoplasm towards the poi
. The cell in development and inheritance. Cells. 146 FERTILIZATION OF THE OVUM enters (Fig. 74). Thus, the starfish Q^g, according to Fol, pos- sesses before fertilization a pecuhar protoplasmic " attraction-cone " to which the head of the spermatozoon becomes attached, and through which it enters the Qgg. In some of the hydromedusse, on the other hand, the entrance point is marked by a funnel-shaped depression at the egg-periphery (Metschnikoff). When no preformed attraction- cone is present, an " entrance-cone " is sometimes formed by a rush of protoplasm towards the point at which the spermatozoon strikes the ^gg and there forming a conical elevation into which the sperm- head passes. In the sea-urchin (Fig. 74) this structure persists only a short time after the spermatozoon enters, soon assuming a /- m n â â -â a. lilO . â Fig. 74. â Entrance of the spermatozoon into the egg. In the sea-urchin Toxppneustes. //. In the medusa il/z/wcowi?. [Metschnmkoff.] /. In the star-fish .-:/.f/tV7aj. [FoL.] A. Spermatozoon of Toxopneustes, X 2000; a, the apical body, ;/, nucleus, m, middle-piece, f, fiagellum. D. Contact with the egg-periphery. C. D. Entrance of the head, formation of the entrance-cone and of the vitelline membrane (y), leaving ihe tail outside. Later stages. G. Appearance of the sperm-aster {s) about 3-5 minutes after first contact; entrance-cone break- ing up. H. Entrance of the spermatozoon into a preformed depression. /. Approach of the spermatozoon, showing the preformed attraction-cone. ragged flame-shape and breaking up into slender rays. In some cases the remains naked, even after fertilization, as appears to be the case in many ccelenterates. More commonly a vitelline mem- brane is quickly formed after contact of the spermatozoon, â in Amphioxiis, in the echinoderms, and in many plants, â and by means of this the entrance of other spermatozoa is prevented. In eggs surrounded by a membrane before ferti
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
