DISTOMUM HEPATICUM 9 



and as many as two hundred flukes having been found in the 

 liver of a single sheep, the total number of eggs passed out of 

 the animal would amount to a hundred millions. 



So long as the eggs remain in the body of the sheep they do 

 not undergo any further development, but they are carried by 

 the bile into the intestine, and pass to the exterior with the 

 droppings. Then, under favourable conditions, they develop 

 further. The conditions necessary for development are mois- 

 ture and a certain degree of heat, a temperature of about 75 

 Fahr. being the most favourable. The eggs will not develop if 

 kept at the normal temperature of the mammalian body, and 

 they develop but slowly at a temperature of ,60 Fahr. At 

 lower temperatures they undergo no change at all, and it follows 

 that they will not develop in winter time, but only in warm 

 weather in the summer months. 



The proper conditions of moisture and warmth being sup- 

 plied, segmentation proceeds, and gives rise to a ciliated 

 embryo of the form shown in fig. 2 (2). During segmentation 

 the growing embryo is nourished by the vitelline cells con- 

 tained in the egg case, and when fully formed it lies curled up 

 in the shell, but presently forces off the operculum and emerges 

 into the water as the larval form. 



This larva, sometimes called a miracidium, is conical in 

 shape, and covered all over with cilia, by means of which it 

 swims actively through the water. In swimming, its broader 

 end is directed forward, and this anterior extremity is furnished 

 with a median projection which may be protruded as a conical 

 spike, or retracted so as to form a central knob or papilla. The 

 external skin of the larva is formed of five closely fitting rings 

 of large hexagonal cells, whose relations are clearly shown in 

 the figure. The most anterior ring comprises four, more rarely 

 five cells, which are thicker and more rounded than the others. 

 The second ring is formed of five or six more flattened cells, 

 the third and fourth rings of four cells each, and the last row 

 of only two cells. All these cells are richly ciliated. 



Beneath the ciliated cells is a granular layer containing 

 nuclei, but cell outlines can hardly be distinguished in it. 

 This layer is probably homologous with the external layer or 

 ectoderm of the adult fluke. It contains a circular and a longi- 

 tudinal layer of muscle fibres, and also a sense organ in the form 

 of a double eyespot, and a pair of excretory organs or nephridia. 



