36 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



recline, (4) A number of tadpole-shaped cercarice are formed in 

 the redia by budding. The cercaria makes its way out of the 

 siiail, and after losing its tail, encysts upon grass. (5) A young 

 fluke is formed within the cyst from the remains of the cercaria. 

 (6) If the cyst is now swallowed by a sheep the young fluke 

 emerges, makes its way up the bile-duct into the liver, and be- 

 comes sexually mature. 



Cleavage (segmentation), which occurs within the oviduct, is 

 regular and complete. The ovum, together with a number of 

 yolk-cells, is enclosed in a horny shell to form the egg. This is 

 oval, with a smooth surface. The ovum lies at one end, and a 

 small circular area of the transparent shell is here marked off as 

 the lid or operculum. The further development occurs outside 

 the body of the sheep, the eggs passing to the exterior with the 

 excrement. It leads to the formation of a free embryo (a and b) t 

 which gradually comes to occupy most of the cavity of the egg, 

 as the yolk-cells are used up. When fully developed, a sudden 

 elongation of its body causes the operculurn to fly open, and it is 

 thus liberated. The body is somewhat conical, and its thick an- 

 terior end possesses a short retractile head-papilla. The external 

 layer of the body-wall is formed by flattened ectoderm cells which, 

 except on the head-papilla, bear long locomotor cilia. The 

 deeper layer of the body-wall is granular, with rudimentary 

 muscle-fibres, a pair of excretory funnels, and near the anterior 

 end a pair of eye-spots. These are two small refracting cells, 

 placed close together, and each containing a crescentic mass of 

 pigment. The two crescents placed back to back present a 

 somewhat X- shaped appearance. The eye-spots are imbedded 

 in an ectodermic thickening, the cerebral ganglion. The interior 

 of the body is mostly filled with rounded germinal-cells, but there 

 is also an oval mass of endoderm cells (b, D), representing a rudi- 

 mentary and mouthless gut (digestive tube). The germinal and 

 other cells which come between this and the ectoderm are collec- 

 tively known as the mesoderm (mesoblast). 



If the free embryo finds itself in water or among damp herbage, 

 it moves actively about by means of its cilia, and should it meet 

 with a small water-snail, Limnaea truncatula, within about eight 

 hours development proceeds, but not otherwise. The head-papilla 

 is lengthened, and the embryo, revolving rapidly, bores by its 

 means into the snail. Within this host, generally in the lung- 



