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 external relations of the animals ; such 

 as locomotor, prehensile, food -receiving, protective, aggressive, and 

 copulatory organs. Of internal 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, i.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 (i) 

 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 generate, 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 cellular structure and origin. The 

 simplest animals (Protozoa) are typically single cells or unit 

 masses of living matter ; as such all animals begin ; but all, 



