40 THE ELEMENTS OF STRUCTURE 
clefts which persist in embryonic reptiles, birds, and 
mammals, though most of them serve no obvious purpose, 
or the embryonic teeth of whalebone whales. These are 
“vestigial structures,” traces of ancestral history, and in- 
telligible on no other theory. The gill-clefts are used for 
respiration in all vertebrates below reptiles; the ancestors 
of whalebone whales doubtless had functional teeth. 
Classification of organs.—We may arrange the various parts of the 
body physiologically, according to their share in the life. Thus some 
parts have most to do with the ex¢ermal/ relations of the animals ; such 
as locomotor, prehensile, food-receiving, protective, aggressive, and 
copulatory organs. Of zz¢ernal parts, the skeletal structures are passive ; 
the nervous, muscular, and glandular parts are active. The repro- 
ductive organs are distinct from all the rest. They are conveniently 
called ‘‘ gonads,” which is a better term than reproductive glands. 
For by a gland we mean an organ which secretes, whose cells produce 
and liberate some definite chemical substance, such as a digestive 
ferment ; whereas the gonads are organs where there is periodic multi- 
plication of certain cells, kept apart from the specialisation character- 
istic of most of the ‘‘body cells” or ‘‘somatic” cells, It is true, 
however, that an accessory glandular function is often associated with 
the gonads. 
Another classification of organs is embryological, z.e. according to the 
embryonic layer from which the various parts arise. Thus the outer 
layer of the embryo (the ectoderm or epiblast) forms in the adult—(1) 
the outer skin or epidermis ; (2) the nervous system ; (3) much at least 
of the sense organs: the inner layer of the embryo (the endoderm or 
hypoblast) forms at least an important part (the ‘‘ mid-gut ”) of the food 
canal, and the basis of outgrowths (lungs, liver, pancreas, etc.) which 
may arise therefrom, and also the notochord of Vertebrates: the middle 
layer of the embryo (the mesoderm or mesoblast) forms skeleton, 
connective swathings, muscle, lining of body-cavity, etc. 
III. Tissues.—Zoological anatomists, of whom Cuvier 
may be taken as a type, analyse animals into their com- 
ponent organs, and discover the homologies between one 
animal and another. But as early as 1801, Bichat had 
published his Anatomie générale, in which he carried the 
analysis further, showing that the organs were composed of 
tissues, contractile, nervous, glandular, etc. In 1838-39, 
Schwann and Schleiden formulated the “cell theory,” in 
which was stated the result of yet deeper analysis—that all 
organisms have a cel/wlay structure and origin. The 
simplest animals (Protozoa) are typically single cells or unit 
masses of living matter; as such all animals begin; but all, 
