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 ■■-■ 

 a 



lilO . ■ 









Fig. 74. — Entrance of the spermatozoon into the egg. A.-G. 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. E.F. 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 Q.gg 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, — e.g. 

 in Amphioxiis, in the echinoderms, and in many plants, — and by 

 means of this the entrance of other spermatozoa is prevented. In 

 eggs surrounded by a membrane before fertilization, the spermato- 

 zoon either bores its way through the membrane at any point, as is 

 probably the case with mammals and amphibia, or may make its 

 entrance through a micropyle. 



In some forms only one spermatozoon normally enters the ovum, 



