108 A. P. THOMAS. 



exceedingly large, and its fecundity has been underrated. 

 In one case I obtained 7,400,000 from the gall-bladder 

 of a sheep suffering from the rot, and, as the liver contained 

 about 200 flukes, this gives an average of 37,000 eggs to each 

 fluke. And these eggs were found in the gall-bladder alone ; 

 the liver must have contained at least as many more, and eggs 

 had been passed copiously by the sheep for several months. 

 The number of eggs produced by a single fluke may be safely 

 estimated at several hundred thousands. 



When first formed, the egg includes a single germinal cell, 

 supplied by the germarium, and fertilised by a spermatozoon, 

 and a considerable number of secondary yolk-cells supplied by 

 the vitellaria, vi^hich serve as food to the growing embryo. 

 Segmentation of the ovum takes place during the descent of 

 the egg through the oviduct, but no further development takes 

 place so long as the egg remains with the body of the host. 

 Fig. 1 on Plate II represents a fluke egg in the condition in 

 which it is found within the bile ducts of the sheep; the 

 embryo is represented by a pale spherical mass of delicate 

 nucleated cells, and is situated near the opercular end of the 

 shell. It is surrounded by the secondary yolk-cells, which are 

 filled with refractive spherules, both large and small, so that 

 the examination of the embryo is rendered very difficult. 



The further development of the embryo can only take place 

 out of the body of the bearer of the adult fluke and at a lower 

 temperature. Eggs kept in an incubator, at the temperature of 

 the mammalian body, do not make any progress, whilst the 

 eggs kept at a lower temperature complete their development 

 in a few weeks. The conditions necessary for development 

 are moisture and a certain moderate degree of warmth. Light 

 I have found to exert no influence ; eggs taken from the gall- 

 bladder and placed directly with water into an opaque vessel, 

 develop as soon as similar eggs exposed to light but otherwise 

 kept under the same conditions. A temperature of about 23° 

 C. to 26° C. is most favorable, and with this degree of 

 warmth the embryo is formed in about two or three weeks. 

 At a lower temperature development takes place much more 



