52 



The Rot in Sheep. 



according to Verloren, this is completed in one species of Ascaris 

 within a few weeks, the eggs of the Ascaris lumbricoides require 

 at least eleven to twelve months for the purpose. Even 

 Richter's first statement spoke of such a period : according to a 

 communication from him in January, 1857, embryos had then 

 begun to be formed in eggs which had been put into water by 

 him in February, 1856, but they did not move." * 



It is difficult to say under what circumstances the embryos of 

 the future distomata will be most quickly matured, so as to 

 escape from the ova. The nearer, however, all experiments to 

 determine a point like this are made to approximate the natural 

 order of things, the greater will be their value. In illustration 

 of this position we shall now describe the experiment previously 

 alluded to, which was begun on January 17, 1853. 



Reflection led us to adopt the following plan for keeping the 

 ova damp only, while at the same time they were being freely 

 exposed to the atmosphere — imitating in this respect their 

 location on a wet pasture. Two or three layers of bibulous 

 paper were floated on water in an ordinary soup-plate, and upon 

 these were sprinkled some ova obtained fresh from the biliary 

 ducts of a rotten sheep. They were carefully examined day 

 by day, and after a short time it was evident that the developing 

 process was quickly going on in the interior of many of them. 



On the 1st of March we detected, for the first time, some of 

 the ova without an operculum, and a number of free nucleated 

 cells (embryos) identical with those previously described. By 

 the 10th of the month more ova had parted with their operculum, 

 and the number of embryos had consequently increased. Poly- 

 gastric monads, of the form previously described, had also made 

 their appearance, and apparently bore a proportion to the early 

 liberated embryos. Matters thus continued throughout the 

 month, and into May, but without any variation of sufficient 

 importance for the further continuance of the experiment. By 

 far the greater part of the ova were even at this time as perfect 

 in their form as when originally placed upon the wetted paper. 



In his recently published book on Parasites (' Entozoa in 

 Man and Animals,' 1879) Dr. Cobbold says that 



" At the time of their expulsion the eggs exhibit a finely segmented con- 

 dition of the yolk. 



" The egg-contents continue to develop whilst outside the parent's body, 

 the granular matrix finally becoming transformed into a ciliated embryo, 

 which when set free follows the habit of infusorial animalcules in general by 

 swimming rapidly in the water. The escape of the embryo is effected at the 

 anterior pole of the egg-shell, which is furnished with a lid that opens in 



* * Animal and Vegetable Parasites.' Translated bv Dr. Lankester. London, 

 1847. 



