The last chapter in this life-history is that in which the sheep crops the blade of grass on which the parasite lies encysted; whereupon the cyst is dissolved in the stomach of the host, the little liver-fluke becomes active, passes through the bile-duct into the liver of the sheep, and there, growing rapidly, reaches sexual maturity, and lays its thousands of eggs, from each of which a fresh cycle may take its origin. The sequence of phenomena is characterized by discontinuity of development. Instead of the embryo growing up continuously into the adult, with only the atrophy of provisional organs (e.g. the gills and tail of the tadpole, or embryo frog), it produces germs from which the adult is developed. Not merely provisional organs, but provisional organisms, undergo atrophy. In the case of the liver-fluke there are two such provisional organisms, the embryo sporocyst and the redia. We may summarize the life-cycle thus-- 1. _Ovum_ laid in liver of sheep, passes with bile into intestine, and thence out with the excreta. 2. _Free ciliated embryo_, in water or on damp earth, passes into pulmonary cavity of _Limnæus truncatulus_, and develops into 3. _Sporocyst_, in which secondary embryos are developed, known as 4. _Rediæ_, which pass into the digestive glands of _Limnæus_, and within which, besides daughter rediæ, there are developed tertiary embryos, or 5. _Cercariæ_, which pass out of the intermediate host and become 6. _Encysted_ on blades of grass, which are eaten by sheep. The cyst dissolves, and the young flukes pass into the liver of their host, each developing into 7. A _liver-fluke_, sexual, but hermaphrodite. Here, again, we notice that one fertilized ovum gives rise to not one, but a number of liver-flukes. We must now pass on to consider the growth and development of organisms. Simple growth results from the multiplication of similar cells. As the child, for example, grows, the framework of the body and the several organs increase in size by continuous cell-multiplication. Development is differential growth; and this may be seen either in the organs or parts of an organism or in the cells themselves. As the child grows up into a man, there is a progressive change in his relative proportions. The head becomes relatively smaller, the hind limbs relatively longer, and there are changes in the proportional size of other organs. In the development of the embryo from the ovum, the differentiation is of a deeper and more fundamental character. Cells at first similar become progressively dissimilar, and out of a primitively homogeneous mass of cells is developed a heterogeneous system of different but mutually related tissues.