LIFE HISTORY OF THE LIVER FLUKE. 119 



papilla and the two separated eye-spots, whilst the other is, of 

 course, without the signs of any such structures. Hence sporo- 

 cysts can be found, which even at an early stage show no trace 

 of head-papilla or eye-spots ; and in the majority of adult 

 sporocysts these organs have degenerated entirely. 



B. 1. Development ofRedia within Sporocyst. — It has 

 been mentioned above that the germinal cells which give rise to 

 the redise are in part already present in the embryo, but that 

 they gain an increase in their numbers by the proliferation of 

 cells lining the body cavity. The earliest stages in the develop- 

 ment of the spores cannot be so well distinguished in the sporo- 

 cyst as in the redia. The cells within a sporocyst having the 

 length of about "2 mm., begin to show an arrangement into 

 rounded masses or solid morulae. One side of the morula then 

 becomes flattened, and the cells here then appear to be invagi- 

 nated, producing a gastrula, whilst the surface becomes smooth, 

 and its outline first round and then oval. The cells forming the 

 opposite sides of the archenteron are in contact, so that there is 

 no archenteric cavity. Each spore may now be seen to be sur- 

 rounded by a delicate membrane, and as it increases in size its 

 form becomes more nearly quadrate. At one end a number of 

 cells are separated to form a spherical pharynx leading into the 

 blind digestive tract, which now extends a little beyond the 

 middle of the body. A little behind the pharynx, the body 

 shows a slightly raised annular ridge, whilst more posteriorly 

 two short blunt processes are formed. Germs, as described 

 above, and in various stages of development, are found in each 

 mature sporocyst ; there is usually one redia (or less frequently 

 two), nearly ready to leave the sporocyst, with two or three 

 germs of medium size, and several small ones. Owing to the 

 varying size and shape of the included germs, the sporocysts 

 have frequently a very irregular outline. 



As soon as the redia is ready to issue from the sporocyst, 

 which is usually the case by the time it has reached the length 

 of '26 mm., it shows active movements, which increase in 

 strength until at length it succeeds in rupturing the wall of the 

 sporocyst, and as this state of contraction is continued, the 



