480 THE HORSE'S POSITION IN THE ANIMAL WORLD 



and at a stage somewhat later, its structure is like the structures displayed 

 at corresponding phases by a less extensive multitude of organisms. At 

 each subsequent stage features are acquired which successively distinguish 

 the embryo from groups of embryos which it previously resembled, thus 

 step by step diminishing the class of embryos which it still resembles, 

 and finally the class is narrowed to the species of which it is a member. 

 The embryo of a bird or a rabbit has at one time in its development 

 characters resembling those of the embryo of the fish — structures repre- 

 senting gill-clefts, for example. In the human embryo, it is only after 

 exhibiting successive changes characteristic of the organization of lower 

 animals that it at last assumes the form proper to man. 



To the naturalist many instances will readily occur of remarkable 

 changes of form during the evolution of an animal from the ovum to 

 the mature stage. Steenstrupp, the Danish naturalist, in 1845 summarized 

 the jjrocess of develojDment in the Medusae, Entozoa, and others of the 

 lower animals, under the title of " Alternation of Generations ", which 

 he described as " the remarkable and till now inexplicable natural pheno- 

 menon of an animal producing an offspring, which at no time resembles 

 its parent; but which, on the other hand, itself brings forth a progeny 

 which returns in its form and nature to the parent animal; so that the 

 maternal animal does not meet with its resemblance in its own lirood, but 

 in its descendants of the second, third, or fourth generation." This re- 

 markable form of evolution is exhibited in the reproductive process of the 

 parasite the liver Huke [Distoma hepaticum) in the most striking manner. 

 The parent fluke provides the ovum, and there its responsibility seems to 

 cease. Hatching takes place in any moist spot or stagnant pool. The 

 product, however, is not a young fluke, but a long, thin embryo, having no 

 resemblance to the parent. Soon, however, this undergoes a change into 

 a cyst, or sort of bag, in the interior of which are developed more advanced 

 organisms known as reclia, and in them, again, still more advanced tailed 

 cercaria appear, which are nearest to the form of the fluke, and only await 

 entrance into the body of a warm-blooded animal to acquire their perfect 

 form and thus prove their descent from their original parent. (See p. 260 

 of this volume.) 



Equally remarkable transformations occur during the development of 

 the embryo in the higher animals, but these go on within the organism 

 of the parent, or otherwise while the young animal is enclosed in the shell 

 of the egg. It is, however, possible to imagine that the changes which 

 occur in the embryo, which is hidden from sight, as in the egg of the bird or 

 the uterus of the mammalian, might be displayed to view, as it is in some of 

 the Entozoa and other animals lower in the .scale of life. AVhat a wonderful 



