56 



ZOOLOGY 



SECT. 



to fill up its cavity. The inner or mesial face of each pouch is 

 covered by atrial epithelium pushed inwards by the growth of 

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

 ductive organ, is a single layer of epithelium which is shown 

 by development to be coelomic. Hence each gonad is surrounded 

 by a closed coelomic sac. 



B 



D 



FIG. 749. Amphioxus lanceolatus. Segmentation of the oosperm. D, the four-celled 

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

 (From Korschelt and Heider, after Hatschek.) 



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

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

 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. After maturation (Fig. 749, A) and impregna- 

 tion, the follicular membrane separates from the oosperm, 



