LESS, xxvi DEVELOPMENT 295 



covered with ccelomic epithelium : by differentiation of groups of cells 

 of the latter the spermaries (Spy] are formed. 



B, a single sperm, showing expanded head and delicate tail. 



c, horizontal section of a sexually mature female. 



The body- wall (Ctt, Dcr. Epthm, M. PI] has undergone partial 

 histological degeneration, and is ruptured in two places to allow of the 

 escape of the ova (pv) which still fill the coelomic spaces enclosed between 

 the body-wall, the enteric canal (Ent. Epthm}, and the septa (Scp). 

 (After Fraipont.) 



% 



B : see p. 255) : in the female they enlarge immensely, and 

 take on the character of ova (c, 0v). Multiplication of the 

 sexual products takes place to such an extent that the whole 

 coelome becomes crammed full of either sperms or ova (c). 



In the female the growth of the eggs takes place at the 

 expense of all other parts of the body, which undergo more 

 or less complete atrophy : the epidermis, for instance, be- 

 comes liquefied and the muscles lose their contractility. 

 Finally rupture of the body-wall takes place in each segment 

 (c), and through the slits thus formed the eggs escape. So 

 that Polygordius, like an annual plant, produces only a 

 single brood : death is the inevitable result of sexual 

 maturity. Whether or not the same dehiscence of the 

 body-wall takes place in the male is not certain : it has 

 been stated that the sperms make their escape through the 

 nephridia. 



Thus while there are no specialized gonaducts^ or tubes for 

 carrying off the sexual products, it is possible that the ne- 

 phridia may, in addition to their ordinary function, serve 

 the purpose of male gonaducts or spermiducts. Female gona- 

 ducts or oviducts are however entirely absent. 



The ova and sperms being shed into the surrounding water, 

 impregnation takes place, and the resulting oosperm under- 

 goes segmentation or division (see p. 248), a polyplast being 

 formed. By the arrangement of its cells into two layers and 



