TREMATODES : DEVELOPMENT 



223 



that of the female organs. It is only later with the onset of egg forma- 

 tion that the uterus is fully developed. Copulation, however, takes place 

 also in the case of fully grown forms with completely developed uteri. 



(2) Formation of the Ova. The ovarian cells arising from the ovary 

 first become mature after their entry into the ootype by the formation 

 of three polar bodies, fertilization then taking place. At the same 

 time as the ovarian cell a number of yolk cells from the vitellarium 

 and secretion, drop by drop, from the shell gland reach the ootype. 1 

 The shell is then formed during the generally active contractions of 

 the ootype walls and then passes on into the uterus. In the uterus of 

 the endoparasitic trematodes the eggs accumulate more and more, 

 often in large quantities, while in ectoparasitic species generally only 



FlG. 128. Ovum of Fasciola 

 hepaiica, L., cut longitudinally. 

 The lid has been lifted in the 

 process. Within the egg are 

 numerous yolk cells, and at the 

 lid end there is the still unseg- 

 mented ovum (dark). 240/1. 



FIG. 129. Miracidium of 

 Fasciola hepatica that has just 

 hatched from the egg, with a dis- 

 tinct cuticular ciliatedepithelium. 

 Magnified. (From Leuckart.) 



one or some few eggs can be found. The completed ova are of 

 various forms and sizes. They are mostly oval, at all events in the 

 digenetic trematodes, and the yellowish or brown shell is provided 

 with an opening at one pole which is closed by a watch-glass-shaped 

 lid (operculum). Appendages (filaments) on the shell at one or both 

 poles are uncommon, but are the rule in the ova of the Monogenea 

 (ectoparasitic species). 



(3) Deposition of the Ova. Soon after their formation, the 

 Monogenea (ectoparasitic trematodes) deposit round the place of 



1 [Recent work (e.g., Goldschmidt, Zool. Anzeiger, xxxiv, p. 482) has shown that the 

 older views regarding the formation of the egg must be modified. In certain species, at any 

 rate, the shell material is formed by the yellow droplets of the yolk glands and not by the so- 

 called shell gland (Mehli's gland) secretion, which is clear and watery. The function of this 

 secretion accordingly still requires explanation ; according to Looss it serves as a covering 

 secretion for the egg-shell proper. It appears also that other granules, the yolk granules as 

 distinct from the shell drop granules, are not always used up during the development of the 

 embryo and hence do not function as yolk, so these also when they exist, and frequently 

 they are wanting, must serve some other purpose, possibly that of imbibing water for the use. 

 of the embryo. J. W. W. S.] 



