DEVELOPMENT OF THE NEMATODES 371 



DEVELOPMENT OF THE NEMATODES. 



After impregnation, the ovum develops around itself a delicate 

 membrane (vitelline membrane), and subsequently an egg-shell is 

 formed. This is derived either as a secretion from the uterine wall 

 or it is a further differentiation of the vitelline membrane, the origi- 

 nal single membrane splitting into two, the outer becoming the egg- 

 shell. Further the uterus often secretes a special albuminous covering 

 around the egg-shell. The "yolk" granules of the ovum are secre- 

 tions of the protoplasm of the ovum itself and first appear when the 

 rhachis is formed. In certain cases ova lie in follicles or capsules 



sp. 



si. 



Dil. 



F. 



FIG. 268. Hind end of a male Ascaris lumbricoides cut across at the level of the dilator 

 cells of the gut. D., gut; DiL, dilator cells of the gut; F., a process of the dilator cells 

 forming a network over the vas deferens; St., lateral line; Sp., spicule ; Vd., vas deferens. 

 The anterior end of the worm lies to the right. Magnified. (After Goldschmidt.) 



formed of epithelium cells derived from the ovarian tubes. These 

 cells subsequently fuse and form a membrane the CHORION. 



The shape of the completed eggs is characteristic of the different 

 species, and therefore a single egg often suffices to diagnose the 

 species. According to the species, the eggs may be deposited sooner 

 or later, either before or during segmentation, or with the embryo 

 perfectly developed. Only a few species are viviparous, e.g., Dmcun- 

 ailus iiiedinensis, Trichinella spiralis ; in the other Nematodes the 

 further development of the extruded eggs takes place after various 



