Reproduction and Development. 49 



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 Limnceus truncatulus, and 

 develops into 



3. Sporocyst, in which secondary embryos are developed, 

 known as 



4. Eedice, which pass into the digestive glands of 

 Limnceus, and within which, besides daughter rediae, there 

 are developed tertiary embryos, or 



5. Cercaria, 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. Develop- 

 ment 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 



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