50 



The Rot in Sheep. 



yielded even by one fluke during its abode in the bile ducts of 

 a sheep exceeds any estimate which the mind seems capable 

 of forming. Examined microscopically, the ova are of them- 

 selves very interesting objects, apart from any knowledge we 

 may have of their appointed end. The annexed illustration 

 (fig. 9) very faithfully depicts their appearance when viewed 



Fig. 9. 



Ova of the Liver-fluke, showing the manner of the escape of their contents by the 

 detachment of the Opercula. Magnified. 



in the field of the microscope. It not only represents their 

 form, but shows the relative size of their contents, and the 

 manner in which these made their escape. Most of the ova 

 measure about -j-^ of an inch long, and 3-^- of an inch broad ; 

 but they are liable to slight modification, some being rather 

 larger than others. To the unassisted vision each egg may be 

 made distinctly visible, by putting a number in a small phial 

 filled with water, agitating this, and then watching their fall 

 while holding it to the light. Their being rendered so percep- 

 tible by this procedure is doubtless due in part to their yellowish- 

 brown colour. 



The density of the shells of the ova is probably an important 

 means for enabling them to resist mechanical injuries or decom- 

 position, and to retain their vitality for a much longer period 

 than otherwise would be the case. How long their vital power 

 may continue it is even impossible to conjecture. We have 

 kept ova in water for upwards of two years — exposed during 

 the whole time to the air by leaving the cork out of the bottle — 

 without observing any very great change in the majority of 

 them. Nothing at all approaching to decomposition could be 

 detected, but whether all of them retained their vitality could 

 not be determined. That some, however, had done so is evident 

 from the result of the following experiment, which was begun in 

 January 1853, simultaneously with another, hereafter to be 

 described, and was continued to April 1855. 



On September 28, 1853, eight months subsequently to placing 

 the ova in water, a few of them were noticed to have parted 

 with the operculum, and many circular, nucleated cells were 

 detected in the fluid. These were of somewhat larger size, but 

 otherwise identical with others still present in the interior of 

 many of the ova. They had a tremulous motion, which was 



i 



