166 ENTOZOA. 



departure from the maternal organs, the eggs display an average 

 longitudinal diameter of tbo", whilst their greatest transversal mea- 

 surement is about 2^0 • They are, therefore, of no inconsiderable 

 size, although, as is estimated by Leuckart, the uterus of a fiiU- 

 grown fluke may contain as many as 45,000 separate ova, at any 

 one given time. When ready for expulsion the contents of the ova 

 display an irregular granular matrix, in which a number of variously- 

 sized oil-like globules are conspicuous, the whole contents being 

 surrounded by a thin yelk-membrane. According to my own obser- 

 vations made on ova placed in water (January 6, 1863), the contents 

 soon afterwards acquire a more definite shape, but did not arrive at 

 an imperfect stage of embryonic development until the expiration of 

 about six weeks (March 16). Leuckart, however, has watched 

 these changes more fully, and has traced them much further than 

 myself. He states, as a general proposition, first, that " the eggs of 

 the hver-fluke continue their development outside the maternal body 

 in water, each finally secreting an embryo, which, after the manner 

 of other distomes, swims about freely by the assistance of a uni- 

 formly-developed, ciliated covering."* Distinct peripheral cells 

 having been formed out of the contained granular mass, the embryo 

 first makes its appearance in the form of a more or less oval, cen- 

 trally-placed, well-defined body. As growth proceeds, the peri- 

 pheral cells break down, their place being occupied by the charac- 

 teristic oil-like yelk globules, which assume every degree of size, 

 and by a gradual coalescence ultimately form a transparent mass 

 almost as large as the embryo itself. After a while, the embryonic 

 body elongates and displays traces of segmentation, in which stage 

 it thus exhibits an anterior section capped by a small papillary 

 eminence, a cephalic or second segment supporting a clearly- 

 defined pigment spot, or eye, in the form of a cross, and a third or 

 body segment constituting one half of the entire embryo. Until 

 water had been permitted to gain access to the interior of the shell, 



* " Die Mun^chlifhen Parasitcn," s. 564. 



