168 



GENERAL EMBRYOLOGY 



cone (Wilson). This not only rises to meet the spermatozoon, 

 but seems to aid in drawing it into the egg (Fig. 85). In some 

 instances (e.g., Julus) this attraction cone may be quite high 

 and may contain a part of the chromatic substance of the egg 

 nucleus. According to the observations of Lillie, the sper- 

 matozoon of Nereis is clearly drawn into the egg through 

 the activity of the latter, the sperm itself taking no active 

 part in the process (Fig. 86). 



- 

 a 



FIG. 85. Entrance of the spermatozoon into the egg. From Wilson, " Cell," 

 H, after Metschnikoff ; /, after Fol. A. Spermatozoon of Toxopneustes, X 2000; 

 a, the apical body; n, nucleus; ra, 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. In some other 

 Echinoderms, the tail may enter the ovum. E,F. Later stages. G. Appearance 

 of the sperm-aster (s) about three to five minutes after first contact; entrance- 

 cone breaking up. H. Entrance of the spermatozoon into a preformed depres- 

 sion. /. Approach of the spermatozoon, showing the attraction-cone. 



When the egg is surrounded by membranes of some thickness 

 or density, the spermatozoa are usually unable to penetrate 

 them and the only path of entrance is then through the micro- 

 pyle, the existence of which is an adaptation for this event. 

 There is apparently no agent directing the sperm toward this 

 perforation in the membranes; the finding of it is a matter of 

 chance. The chance is not small, however, that some sperma- 

 tozoon will enter the micropyle, for ordinarily the fluids around 

 the egg are filled with a swarm of sperm cells. As already 

 noted the micropyle is commonly at the animal pole of the 

 egg, though at the vegetal pole in a few instances (some 



