LIFE HISTOKY OF THR LIVER-FLUKE. 115 



papilla, persist, showing the identity of the young sporocyst 

 with the embryo of the liver-fluke. After the change in form 

 has taken place the length is only ahout -07 mm. The rudi- 

 mentary digestive tract remains for a time, but later on is no 

 longer distinguishable. The growth of the various larval forms 

 of trematodes depends very much on the temperature to which 

 they are exposed. During warm summer weather it is very 

 rapid, and in the case of Fasciola hepatica, the sporocyst , 

 may reach its full size before the end of a fortnight; in autumn 

 development to the same stage takes a period of double the 

 length. The sporocyst commonly preserves the elliptical 

 shape until it reaches the length of '15 mm., after this time the 

 growth is most rapid in the longitudinal direction and the 

 form becomes sac-shaped. The contents of the sporocyst are 

 formed by a number of very clear rounded cells, some of which 

 are the germinal cells of the embryo or cells derived from 

 them by division, others are formed by a proliferation of the 

 epithelium lining the cavity of the sporocyst. If the sporo- 

 cyst be contracted, these cells seem to fill up the whole of the 

 space, and the cells which are still attached to the body wall, ' 

 and form part of its inner surface, cannot be properly distin- 

 guished from those which are lying free. But if a sporocyst 

 be chosen for examination which is not in a state of contrac- 

 tion, cells of various sizes, with very large nuclei, may be seen 

 projecting here and there from the inner surface; sometimes in 

 a single layer, at other times in rounded heaps, two or three 

 cells deep. It is very difficult to follow the earliest stages in 

 the formation of the spores within the sporocyst, but by the time 

 the sporocyst has reached the size of '2 mm., there are always 

 indications that the contents are becoming arranged in separate 

 balls of cells — the germs of the next generation. 



The spoi'ocyst continues to increase in size, and ultimately 

 reaches the length of "5 — "7 mm. (Plate III, fig. 10). On the 

 outer surface is a structureless cuticle, and beneath this is the 

 thin layer in which the external circular and internal longitu- 

 dinal muscle-fibres are often the only structural elements wliich 

 can be distinctly observed ; but in some cases, though not in 



