50 



ZOOLOGY 



SECT. 



covered by atrial epithelium pushed inwards by the growth of 

 the gonad ; within this, and completely surrounding the repro- 

 ductive organ, is a single la) T er of epithelium which is shown 

 by development to be coelomic, Hence each gonad is surrounded 

 by a closed ccelomic sac. 



When ripe the inner walls of the gonadic pouches burst, and 

 the ova or sperms make their way into the atrium and thence 



B 



E 



H 



K 



FIG. 704. Amphioxus lancelolatus. Segmentation of the oosperm. D the four-celled 

 stage, (C) from above ; H, vertical section of G ; K, vertical section of the blastula stage I 

 (From Korscnelt and Heider, after Hatschek.) 



by the atriopore to the external water where impregnation takes 

 place. The laid eggs are covered by a thin follicular membrane, 

 formed of flattened cells. 



Development. Only one polar body has been observed (Fig. 

 704, A). After impregnation the follicular membrane separates 



