Chap. XVII.] THE LIVER-FLUKE AND TAPE-WORM. 327 



and vitelliue ducts. The oviduct is usually crowded with eggs, 

 which are here undergoing the early stages of their development. 

 A canal (Laurer's) from the vitelline duct is said to open dorsally. 



The Ovum. The egg is small (-13 by -08 mm.), oval, smooth, 

 and transparent, with a yellowish-brown chitinous shell. At the 

 anterior end is an operculum (Fig. 97, A., op.}. The embryo (em.) 

 occupies but a small space, the rest being occupied with yolk- 

 cells. Segmentation goes on within the uterus ; but after the 

 egg is laid, further development can only go on at a reduced 

 temperature of 23-26 C. All the eggs laid under the same 

 conditions do not appear to produce embryos at the same time, 

 some embryos emerging weeks, or even months earlier than 

 others. What conditions determine this difference is not known, 

 but such a state of things is clearly of advantage to the species. 

 The embryo within the egg increases by absorption of yolk, 

 develops a head-papilla and a double eye-spot, and becomes 

 ciliated though the cilia are at present motionless. Eventually 

 the operculum springs open and the embryo emerges. 



The Free Embryo. The embryo is conical in form (Fig. 97, C.) 

 with a retractile head-papilla (h.p.) at the anterior end. The whole 

 surface (head-papilla excepted) is covered with long cilia, borne 

 by flattened ectoderm cells arranged in five or six transverse 

 rings around the body. The four cells which compose the first 

 ring are thicker than the others. Beneath the ectodermic ciliated 

 layer is an endodermic body-wall, within which are a number of 

 delicate vesicular germinal cells (g. c.). A rudimentary digestive 

 tract is visible behind the head-papilla. Over this in the figure 

 (after A. P. Thomas) is seen the eye-spot (e. s.\ shaped like two 

 crescents united back to back, and developed in the body-wall. 

 In the body-wall there are also two ciliated funnel-shaped spaces 

 of the excretory system (ex.), in each of which there is a single 

 large flame-shaped cilium. 



The embryo is exceedingly active, darting backwards and 

 forwards, and rotating on its long axis. The duration of its free 

 and active life is, however, not more than some eight hours. If 

 it does not meet with a water-snail within that time it dies. 



