146 



FERTILIZATION OF THE OVUM 



enters (Fig. 74). Thus, the starfish egg, according to Fol, pos- 

 sesses before fertilization a peculiar protoplasmic "attraction-cone" 

 to which the head of the spermatozoon becomes attached, and through 

 which it enters the egg. In some of the hydromedusae, 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 egg 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 



* 



I -\ -* 



: ;.r^>>< 



Fig. 74. Entrance of the spermatozoon into the egg. A.-G. In the sea-urchin Toxopneustes. 

 H. In the medusa Mitrocoma. [METSCHNIKOFF.] /. In the star-fish Asterias. [FOL.] 



A. Spermatozoon of Toxopneustes, X 2000; a, the apical body, n, nucleus, m, middle-piece, 

 / flagellum. B. Contact with the egg-periphery. C. D. Entrance of the head, formation of the 

 entrance-cone and of the vitelline membrane (v), leaving the 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 egg remains naked, even after fertilization, as appears to 

 be the case in many coelenterates. More commonly a vitelline mem- 

 brane is quickly formed after contact of the spermatozoon, e.g. 

 in Amphioxus, 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, 



