110 



PEACTICAL PARASITOLOGY 



will fall out of the uterine folds and these may be examined at once. 

 Young eggs have a colourless shell, old eggs a yellowish-brown one. 

 The contents of both consist of large yolk-cells, which frequently cover 

 the already fertilized, but as yet ungrooved, germ-cell (fig. 31). 



Fig. 30. Fasciola hepatica, L. Ag., Yolk-duct 

 leading from yolk-glands (Dst.). M., Oral sucker. 

 Tr.c., Transverse yolk-ducts leading from yolk- 

 glands. Ut. t Uterus. (Natural size.) 



FIG. 31. Egg of Fasciola hepatica, 

 L. The germ-cell is seen close to 

 the pole at which the lid is placed. 



Careful examination under a strong glass will usually reveal the 

 shell-lid, shaped like a watch-glass and placed at the extremity nearest 

 to the germinal cell. Slight pressure on the cover-glass will cause 

 a certain proportion of the eggs to throw back, or at least to lift, 

 their lids. 



The egs of other endoparasitic Trematodes (figs. 32, 33) may 

 differ from those of the liver-fluke in the following particulars : (1) In 



form ; (2) in size ; (3) in 

 the possession of a thread- 

 like attachment to the 

 shell at one or both poles ; 

 (4) in contents, the egg 

 being deposited at a de- 

 velopmental stage (e.g., 

 with the fully formed 

 miracidium or larva) 



FIG. 32. - 

 Egg of Opis- 

 thorchis fe - 

 linens (Biv.). 

 830 : 1. 



FIG. 33. Egg of Dicrocoslium lan- 

 ceatum, St. et H. On the left, lying 

 on its flat surface; on the right, which will vary with the 

 lying on its side. 



species; (5) in the absence 

 of the shell-lid; and (6) 



in the possession of a thorn-like process at one side, or at one end, 

 of the shell. The eggs of Trematodes are very liable to be mistaken 

 for Coccidia or the eggs of Bothriocephales. 



In the case of the large-hooked worms and the bookless varieties 

 related to them, what are usually described as eggs are finished 

 embryos or oncospheres. These have already lost the true egg-shell, 



