430 CHORD AT A, 



whilst in mammals {q.v.) it takes part in the formation of 

 the placenta. 



Conclusion. — It is clear that the study of comparative 

 anatomy and of development throughout the sub-phylum of 

 Vertebrata can scarcely be over-estimated as a means of in- 

 terpreting the complex and often puzzling structure of the 

 highest vertebrates. 



Certain organs appear to have retained the same function 

 throughout, such as the brain and heart, and we may only 

 trace the lines of growing complexity from a simple tube to 

 the intricate mechanism of such organs as found in man. 

 But others show a still more remarkable history, involving a 

 change of function, which in some instances may almost be 

 regarded as loss of function (though it is daring to assume 

 that an organ can be structurally existent after all function 

 has disappeared). We may recall our teeth traced back to 

 placoid scales, the thyroid and thymus to glandular organs 

 of the atriozoan pharynx, the inner ear to one of a series of 

 aquatic sense-organs, the middle ear to one of a series of 

 visceral clefts and the jaws and the ear-ossicles to parts of a 

 segmented series of visceral arches. 



These and numerous other instances of the same kind 

 teach us that a true knowledge of anatomy can only be 

 obtained by a due appreciation of what we have been as 

 well as what we are. 



[Table. 



