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 



Fig. 749.— AmpMoxus 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 Hcider, after Hatschek.) 



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

 the ova or sperms make their way into the a.trium and thence 

 by the atriopore to the external Avater, where impregnation takes 

 place. The laid eggs are covei'ed 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. 



