

Fig. 143. — Development of Polygordius. — After Fraipont. 



a, Mother sperm cell ; b, c, sperm inorulae ; d, spermatozoa. 



I, Ovum with large nucleus ; 2, two-cell stage ; 3, four-cell stage ; 4, blastosphere ; 

 5, gastrula ; ac, archenteron ; 6, closure of gastrula mouth or blastopore ; 

 7, formation of stomodaeum (st.), and proctodaeum {pr.), which invaginate to meet 

 archenteron (ac.) ; 8, complete gut formed ; 9, elongation of larva ; ap. sp., 

 apical spot ; cil., cihated ring ; neph., primitive nephridia ; 10, formation of 

 posterior segments ; 11, form of adult Polygordius. 



