430 CHORDATA. 
whilst in mammals (g.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 a// 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, 
