296 NEMATHELMINTHES. 



a complicated "uterine bell," which opens into the body-cavity (b), 

 and to the base of which the ligament is attached ; of two short 

 oviducts connecting the bell with the uterus, which leads into the 

 vagina ; the vagina opens at the hind end. There is at the hind 

 end of the bell a second opening into the body-cavity on the so- 

 called dorsal side (c). The female generative ducts consist of a few 

 very large cells, like the cells constituting the skin of the embryo, 

 and the muscles of the ducts are fibrous differentiations of the outer 

 parts of these cells. 



It is only in the young stage that the ovary is a simple body 

 enclosed by the membrane of the above-mentioned ligament. As 

 the animal increases in size, the ovary grows, and becomes divided 

 into numerous spherical masses of eggs, the pressure of which bursts 

 the membrane of the ligament ; the masses of ova, as well as the 

 ripe elliptical eggs, which gradually become free from them, fall into 

 the body -cavity. The egg membranes are not 

 formed till after segmentation, and ought perhaps 

 to lie interpreted as embryonic membranes. The 

 eggs, which already contain embryos, pass out of 

 the body-cavity into the uterine bell, which is 

 continually dilating and contracting, thence into 

 the oviduct, and through the genital opening to 

 the exterior ; while the round still unripe eggs 

 pass from the uterine bell through the dorsal 



FIG. 242. Embryo 



of Eehworhynchus posterior opening back into the body-cavity. 

 gigas enclosed m Development. Segmentation is irregular and 



the egg membranes 



(after Leuckart). complete, and results in the formation of an em- 

 bryo, which, is enclosed in three egg-membranes. 

 The embryo has a small, somewhat long body, armed with small 

 spines at the anterior pole, and consists of a central mass of small 

 cells and a peripheral layer containing a few large nuclei and 

 without cell-limits (Fig. 242). The peripheral layer gives rise to 

 the ectoderm and the lemnisci ; the central cells to the other organs. 

 The large nuclei are said to break up into the small nuclei of the 

 ectoderm of the adult except in Neorliijnclius and Arhynchus. The 

 body cavity arises in the central mass, and the cells on the outside 

 of it form the muscles of the body- wall. In this development there 

 can be no talk of layers in the ordinary sense. The embryo passes 

 into the intestine of Amphipods (E<-li. ja-oteus, polymorphus), or of 

 Isopods (Ech. angustatus), and there becomes free, bores through 

 the wall of the intestine, and after losing the embryonic spines, 



