io6 



DEVELOPMENT OF AMPHIOXUS. 



Ova and spermatozoa are discharged simultaneously by 

 male and female individuals into the water, and fertilisa- 

 tion is effected in the latter medium. 



The final stages in the maturation of the ovum of Am- 

 phioxus are very imperfectly known, and the extrusion of 

 the so-called polar bodies, preparatory to the process of fer- 

 tilisation, has not been properly studied, only one such 



Fig. 53. Fertilised ovum of Amphioxus. Highly magnified. (From a 

 drawing kindly lent by Professor E. B. WILSON.) 



d.c. Directive corpuscle or polar body. o. Ovum. f. Follicle. 



body having been observed, whereas from the analogy of 

 all other sexually reproducing animals we should expect 

 two polar bodies (directive corpuscles) to be given off be- 

 fore the egg was fully mature. As soon as an ovum has 

 been fecundated by the entrance of a spermatozoon, the 

 vitelline membrane springs away from the body of the egg- 

 cell, leaving a wide space between the latter and the 

 former (Fig. 53). This expansion of the vitelline mem- 



