140 



THE ANIMAL PARASITES OF MAN 



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 up by a watch-glass-shaped 

 cover. Appendices (filaments) on the shell at one or both poles 

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

 The latter contain very few or only one ovum in their uterus, 

 whereas in the Digeriea the number of ova is large, often even 

 very considerable ; correspondingly the ova are also very much 

 smaller. 



(3) Deposition of the Ova. Soon after the formation of the ova 

 the Monogenea deposit them round the place of their attachment on 



FIG. 74. Ovum of Fasciola 

 hepatica (L.), cut longitudi- 

 nally. The cover has been 

 removed in the process. 

 Within the shell numerous 

 yolk cells, and at the lid end 

 there is the still unsegmented 

 ovum (dark). 240/1. 



FIG. 75. Miracidium of 

 Distomum hepaticum that has 

 just hatched from the egg, 

 and has a distinct cuticular 

 epithelium (magnified). 

 (From Leuckart.) 



the skin or the gills of their hosts. The embryonic development 

 thus takes place outside the parent ; in other Trematodes the ova 

 remain in the uterus for a longer or shorter period, and in it often 

 go through the entire development or a part of it. Sooner or 

 later the ova are deposited and leave the affected organs by the 

 natural channels. 



(4) The embryonic development, after irregular segmentation, 

 leads to the formation of a morula stage which is surrounded by 

 a cellular investing membrane, while the principal mass of the 

 cell forms the embryo, which uses for its nourishment the yolk 

 cells, which have in the meantime disintegrated. Usually, after 

 the ova have reached the water, the embryos hatch out, leaving 

 their investing membrane in the egg-shell ; in other cases, how- 

 ever, the embryos only hatch out after having been subjected to 

 the influence of the intestinal juices, that is to say, in the intestine 



